Databases and Persistence
backup
noun
Meaning
A copy of a database saved at a certain moment so that the data can be brought back after a failure, a mistake or an attack.
Where you use it
You take, run, schedule, verify and restore backups. Common types are full, differential and incremental backups, and physical versus logical backups. Experienced DBAs repeat one rule: a backup you have never test-restored is not a real backup.
Examples
We take a full backup every night and keep it for thirty days.
Take a backup of the table before you run the migration.
Usage note
The noun is one word (a backup); the verb is two words (to back up the database).
Common phrases
take a backup · restore from backup · full backup · backup schedule · verify a backup
More about this term →NoSQL
noun
Meaning
A broad group of databases that do not use the classic table-and-SQL model, storing data as documents, key-value pairs, wide columns or graphs instead.
Where you use it
Usually read as 'not only SQL'. Well-known examples are MongoDB (document), Redis (key-value), Cassandra (wide-column) and Neo4j (graph). NoSQL systems are chosen for flexible schemas, very high scale or special data shapes; many now support SQL-like queries too.
Examples
We store user sessions in a NoSQL database and keep orders in PostgreSQL.
Choosing NoSQL just because the schema might change is rarely a good reason.
Common phrases
NoSQL database · NoSQL store · SQL vs NoSQL · move to NoSQL
More about this term →constraint
noun
Meaning
A rule set on a table or column that the database enforces automatically, rejecting any data that would break it.
Where you use it
The main kinds are PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, CHECK and DEFAULT (some books do not count DEFAULT as a real constraint). Data that breaks one causes a 'constraint violation' error. You add, drop, name and sometimes temporarily disable constraints during large data loads.
Examples
A CHECK constraint makes sure the discount is between 0 and 100.
Give every constraint a clear name so error messages are easy to understand.
Common phrases
add a constraint · constraint violation · CHECK constraint · drop a constraint · enforce a constraint
More about this term →normalization
noun
Meaning
The process of organizing tables so that each fact is stored in only one place, which reduces repeated data and prevents update mistakes.
Where you use it
A standard topic in courses, interviews and design reviews. It is done step by step through normal forms (1NF, 2NF, 3NF and beyond). People say a schema is 'well normalized' or 'over-normalized', and sometimes choose to denormalize for read speed.
Examples
Normalization moved the customer address out of the orders table into its own table.
The interviewer asked me to explain normalization with a simple example.
Usage note
British English often writes 'normalisation'; database documentation almost always uses the American spelling with 'z'.
Common phrases
database normalization · normalize a table · well normalized · level of normalization
More about this term →ORDER BY
phrase
Meaning
The SQL clause that sorts the rows of a result by one or more columns, in ascending (ASC) or descending (DESC) order.
Where you use it
Written near the end of a SELECT: ORDER BY created_at DESC. Ascending is the default. Without ORDER BY the database does not promise any order, even if results look sorted. It is often used with LIMIT, TOP or FETCH FIRST to get the newest or largest rows.
Examples
Add ORDER BY created_at DESC so the newest tickets appear first.
The test passed locally but failed on CI because the query had no ORDER BY.
Common phrases
ORDER BY ... DESC · ORDER BY ... ASC · sort order · order by date
More about this term →GROUP BY
phrase
Meaning
The SQL clause that puts rows with the same values into groups so that a total, count or average can be calculated for each group.
Where you use it
Used with aggregate functions: SELECT city, COUNT(*) FROM customers GROUP BY city. In standard SQL every selected column that is not inside an aggregate function must appear in GROUP BY; breaking this rule is one of the most common SQL errors.
Examples
Group by month to see how revenue changed over the year.
The query fails because product_name is in the SELECT list but not in the GROUP BY.
Common phrases
GROUP BY a column · group by date · GROUP BY with HAVING · grouped results
More about this term →aggregate function
noun
Meaning
A SQL function that takes many rows and returns a single value from them, such as COUNT, SUM, AVG, MIN or MAX.
Where you use it
Used for reports and dashboards, usually with GROUP BY. Analysts talk about 'aggregating data' and 'aggregates'. Remember that most aggregate functions ignore NULL values, and that COUNT(*) counts rows while COUNT(column) counts only non-NULL values.
Examples
Use an aggregate function like SUM to get the total sales per store.
You can't use an aggregate function in the WHERE clause; filter it with HAVING.
Usage note
As a noun or adjective 'aggregate' ends in a weak '-gut' sound; as a verb (to aggregate) it ends in '-gate'.
Common phrases
use an aggregate function · aggregate query · aggregate data · COUNT(*)
More about this term →inner join
noun
Meaning
A join that returns only the rows that have a match in both tables; rows without a partner on the other side are left out.
Where you use it
The default join: writing just JOIN means INNER JOIN. It is the right choice when every row must have a partner, for example orders with their customers. A common bug is losing rows because an inner join was used where a left join was needed.
Examples
An inner join drops products that have never been ordered.
The report shows fewer customers than expected because of the inner join on addresses.
Common phrases
inner join on · use an inner join · inner join two tables · JOIN ... ON
More about this term →left join
noun
Meaning
A join that returns all rows from the first (left) table and the matching rows from the second; where there is no match, the second table's columns are NULL.
Where you use it
Also written LEFT OUTER JOIN; the two mean the same. It is used to keep rows that have no partner, for example customers with no orders. A classic trick is LEFT JOIN ... WHERE right_table.id IS NULL to find rows with no match.
Examples
Use a left join so customers without any orders still appear in the list.
Putting a condition on the right table in the WHERE clause turns your left join into an inner join.
Common phrases
left join on · LEFT OUTER JOIN · left join with IS NULL · use a left join
More about this term →CREATE TABLE
phrase
Meaning
The SQL statement that makes a new table, defining its name, its columns with their data types and its constraints such as the primary key.
Where you use it
Part of DDL. In real projects it lives in migration files rather than being typed by hand. Useful variants are CREATE TABLE IF NOT EXISTS and CREATE TABLE ... AS SELECT, which copies the result of a query into a new table.
Examples
Paste the CREATE TABLE script here so I can see the column types.
The migration failed because the CREATE TABLE ran twice; add IF NOT EXISTS.
Common phrases
CREATE TABLE statement · CREATE TABLE IF NOT EXISTS · CREATE TABLE AS SELECT · table definition
More about this term →ALTER TABLE
phrase
Meaning
The SQL statement that changes the structure of an existing table, for example to add, rename or drop a column or to add a constraint.
Where you use it
Part of DDL. On small tables it is instant, but on large production tables some ALTER TABLE operations rewrite or lock the whole table, so teams plan them carefully, run them at quiet hours or use online schema change tools.
Examples
The ALTER TABLE locked the orders table for ten minutes during peak hours.
Write an ALTER TABLE to add a NOT NULL column with a default value.
Common phrases
ALTER TABLE ADD COLUMN · ALTER TABLE statement · run an ALTER TABLE · online schema change
More about this term →view
noun
Meaning
A saved SELECT query that you can use like a table. It usually stores no data of its own and shows the current result of the query each time.
Where you use it
Used to hide complex joins, give reporting teams a simple interface or limit which columns a user can see. A materialized view is different: it stores the result physically and must be refreshed. You create, drop and query views.
Examples
The finance team queries a view that joins orders, payments and refunds.
We refresh the materialized view every hour because the base query is expensive.
Common phrases
create a view · materialized view · refresh a view · query a view
More about this term →stored procedure
noun
Meaning
A named block of SQL code saved inside the database that can take parameters and be run again and again with a single call.
Where you use it
Very common in SQL Server and Oracle shops, often shortened to SP or 'proc'. You create, call or execute (EXEC) a stored procedure. Supporters like the speed and central logic; critics say business rules hidden in the database are hard to test and version.
Examples
The month-end report is generated by a stored procedure that runs at midnight.
Don't change that stored procedure; three other systems call it.
Common phrases
call a stored procedure · execute a stored procedure · write a stored procedure · stored procedure parameter
More about this term →ACID
abbreviation
Meaning
Atomicity, Consistency, Isolation, Durability: the four guarantees that make database transactions reliable even with errors, crashes and many users at once.
Where you use it
A favourite interview question. People say a database is 'ACID-compliant' or offers 'full ACID transactions'. Relational databases are ACID by design; many NoSQL systems trade some of these guarantees for scale, although several now offer ACID transactions too.
Examples
We need ACID guarantees for the ledger, so it stays in PostgreSQL.
Can you explain what each letter in ACID means?
Usage note
The 'C' in ACID (data stays valid under the rules) is not the same as the 'C' in the CAP theorem (all nodes see the same data).
Common phrases
ACID-compliant · ACID transactions · ACID guarantees · ACID properties
More about this term →execution plan
noun
Meaning
The step-by-step strategy the database chooses to run a query, showing which indexes it uses, how tables are joined and how many rows it expects.
Where you use it
Also called a query plan. You get it with EXPLAIN in PostgreSQL and MySQL (EXPLAIN ANALYZE also runs the query and shows real times) or with the estimated and actual plan views in SQL Server. Reading plans is a core skill for tuning slow queries.
Examples
The execution plan shows a sequential scan on a ten-million-row table.
Can you share the execution plan from production, not from your laptop?
Common phrases
read an execution plan · query plan · EXPLAIN ANALYZE · actual execution plan · estimated plan
More about this term →query optimizer
noun
Meaning
The part of a database engine that looks at a query and the table statistics and picks the cheapest way to run it, producing the execution plan.
Where you use it
Usually just 'the optimizer'. Most modern databases use a cost-based optimizer that relies on statistics, so outdated statistics lead to bad plans. When the optimizer chooses badly, people update statistics, rewrite the query or, as a last resort, add hints.
Examples
The query optimizer ignored our new index because the statistics were out of date.
After the upgrade, the query optimizer started choosing a different join order.
Common phrases
cost-based optimizer · optimizer statistics · optimizer hint · update statistics
More about this term →replication
noun
Meaning
Keeping copies of the same database on several servers by sending every change from one server to the others, for safety and to share the load.
Where you use it
It can be synchronous (the primary waits for the replica) or asynchronous (faster, but the replica can fall behind; this delay is replication lag). Replication protects against server failure but not against mistakes: a wrong DELETE is copied to every replica, so backups are still needed.
Examples
Replication to the second data center is asynchronous, so we may lose a few seconds of data.
Replication broke last night and the replica is now two hours behind.
Common phrases
set up replication · replication lag · synchronous replication · asynchronous replication · logical replication
More about this term →restore
verb
Meaning
To bring a database back from a backup, either to fix data loss or to create a copy of it on another server.
Where you use it
Also a noun: 'the restore took four hours'. You restore a database, restore from a backup or restore to a point in time. Teams practise restore drills to check that backups work and to measure how long recovery takes.
Examples
We restored last night's backup to a test server to check the missing invoices.
How long would it take to restore the whole database if the disk failed?
Common phrases
restore a database · restore from backup · restore test · restore to a point in time
More about this term →DBA
abbreviation
Meaning
Database administrator: the person responsible for installing, securing, backing up, tuning and keeping a company's databases running.
Where you use it
Used as a job title and in daily talk: 'ask the DBA', 'the DBA team', 'DBA approval'. Production DBAs focus on servers, backups and uptime; development DBAs help with schema design and SQL. Cloud services have taken over some routine DBA tasks.
Examples
Any index change on the billing database needs DBA approval.
She worked as a developer for five years before becoming an Oracle DBA.
Common phrases
ask the DBA · DBA team · on-call DBA · Oracle DBA · DBA approval
More about this term →ER diagram
noun
Meaning
A drawing of a database design that shows the entities (usually tables), their attributes and the relationships between them, such as one-to-many.
Where you use it
ER stands for entity-relationship; the short form ERD is also common. Teams draw ER diagrams before building a schema and generate them from existing databases for documentation. Lines often use crow's foot notation to show 'one' and 'many'.
Examples
Let's sketch an ER diagram before we create any tables.
The ER diagram in the wiki is two years old and missing half the tables.
Common phrases
draw an ER diagram · ERD · entity-relationship model · crow's foot notation
More about this term →one-to-many relationship
noun
Meaning
A link between two tables where one row in the first table can be connected to many rows in the second, but each of those belongs to only one, like a customer and their orders.
Where you use it
The most common relationship in databases. It is built with a foreign key on the 'many' side, for example customer_id in the orders table. Also written 1:N or 1-to-many. ORMs describe the two sides as 'has many' and 'belongs to'.
Examples
There is a one-to-many relationship between departments and employees.
To model a one-to-many relationship, put the foreign key on the child table.
Common phrases
one-to-many relationship between · 1:N relationship · parent and child tables · has many
More about this term →many-to-many relationship
noun
Meaning
A link where many rows in one table can relate to many rows in another, like students and courses. It needs a third table in between to store the pairs.
Where you use it
The table in the middle has many names: junction table, join table, bridge table, link table or associative table. It usually holds two foreign keys that together form its primary key, and sometimes extra data such as the date a student enrolled.
Examples
Products and tags have a many-to-many relationship, so we added a product_tags table.
In an interview, explain how you would store a many-to-many relationship between users and roles.
Common phrases
many-to-many relationship · junction table · bridge table · N:M relationship
More about this term →unique constraint
noun
Meaning
A rule that stops two rows from having the same value in a column or combination of columns, for example two users with the same email address.
Where you use it
Unlike a primary key, a table can have many unique constraints, and most databases let unique columns contain NULLs. Behind the scenes the database builds a unique index. Trying to insert a duplicate gives a 'duplicate key' or 'unique violation' error.
Examples
Add a unique constraint on email so the same person can't sign up twice.
The import stopped at row 812 because of a unique constraint violation.
Common phrases
add a unique constraint · unique index · unique violation · duplicate key error
More about this term →auto-increment
adjective
Meaning
Describes a numeric column whose value the database fills in automatically with the next number in sequence each time a new row is added.
Where you use it
Mostly used for id columns that serve as surrogate keys. The keyword differs by product: AUTO_INCREMENT in MySQL, IDENTITY in SQL Server, SERIAL or GENERATED AS IDENTITY in PostgreSQL, and sequences in Oracle. Gaps in the numbers are normal after rollbacks and deletes.
Examples
The id column is auto-increment, so don't send a value for it in the INSERT.
Exposing auto-increment IDs in URLs lets anyone guess how many orders we have.
Common phrases
auto-increment column · auto-increment ID · identity column · sequence
More about this term →document database
noun
Meaning
A NoSQL database that stores each item as a self-contained document, usually in a JSON-like format, so related data can be kept together without fixed tables.
Where you use it
MongoDB, Couchbase and Firestore are well-known examples. Documents are grouped into collections, and fields can differ from one document to the next. Teams choose them for flexible data, but must think carefully about embedding versus referencing data.
Examples
Product pages with very different attributes fit well in a document database.
We moved the event logs to a document database because their structure keeps changing.
Common phrases
document database · document store · JSON document · embed documents
More about this term →key-value store
noun
Meaning
The simplest kind of NoSQL database, which saves each value under a unique key and finds it again very quickly by that key.
Where you use it
Redis, Memcached, etcd and Amazon DynamoDB are typical examples. Key-value stores are used for caches, user sessions, counters, feature flags and rate limits. You 'get' and 'set' keys and often give them an expiry time (TTL).
Examples
Session data lives in a key-value store, so any web server can read it.
A key-value store is fast, but you can't run complex queries on it.
Common phrases
key-value store · key-value pair · set a key · key expiry · TTL
More about this term →subquery
noun
Meaning
A query written inside another query, in brackets, whose result is used by the outer query, for example as a list of values to filter by.
Where you use it
Also called an inner or nested query. Subqueries appear in WHERE (… IN (SELECT …)), in FROM as a derived table, or in the SELECT list. A correlated subquery refers to the outer query and runs once per row, which can be slow; a join or CTE is often clearer.
Examples
The subquery returns the IDs of customers who ordered last month.
Rewriting the correlated subquery as a join cut the run time in half.
Common phrases
correlated subquery · nested subquery · subquery in the WHERE clause · rewrite a subquery
More about this term →HAVING clause
noun
Meaning
The part of a SQL query that filters groups after GROUP BY has run, usually using the result of an aggregate function such as COUNT or SUM.
Where you use it
The key rule: WHERE filters rows before grouping, HAVING filters groups after grouping. A typical example is HAVING COUNT(*) > 5. Putting a plain row filter in HAVING works but is slower, so it belongs in WHERE. 'WHERE vs HAVING' is a classic interview question.
Examples
Use a HAVING clause to list only the cities with more than 100 customers.
Move the date filter from the HAVING clause to WHERE so fewer rows get grouped.
Common phrases
HAVING COUNT(*) · GROUP BY ... HAVING · filter groups · WHERE vs HAVING
More about this term →SELECT DISTINCT
phrase
Meaning
A form of the SELECT statement that removes duplicate rows from the result, so each combination of returned values appears only once.
Where you use it
Handy for questions like 'which countries do our customers come from?'. Experienced developers are suspicious when DISTINCT is added to hide duplicates caused by a wrong join; the fix is usually the join, not DISTINCT. COUNT(DISTINCT column) counts unique values.
Examples
SELECT DISTINCT country FROM customers gives us the list of markets we sell in.
Don't fix the duplicates with SELECT DISTINCT; the join condition is wrong.
Common phrases
SELECT DISTINCT · COUNT(DISTINCT ...) · remove duplicates · distinct values
More about this term →alias
noun
Meaning
A temporary short name given to a table or column inside a query, usually with the AS keyword, to make the query shorter or the output clearer.
Where you use it
Table aliases like 'FROM customers c' are standard when joining many tables. Column aliases rename output, for example SUM(amount) AS total_sales. Note that in most databases you cannot use a column alias in the WHERE clause of the same query.
Examples
Give the subquery in the FROM clause an alias, otherwise MySQL throws an error.
Use clear aliases like ord and cust instead of a, b and c.
Usage note
Pronounced 'AY-lee-us', with stress on the first syllable.
Common phrases
table alias · column alias · give an alias · AS keyword
More about this term →composite key
noun
Meaning
A key made of two or more columns that identify a row only when used together, such as order ID plus product ID in an order items table.
Where you use it
Typical in junction tables for many-to-many relationships. Some teams prefer a simple surrogate id plus a unique constraint on the column pair, because composite keys make foreign keys and ORM mappings longer. Also called a compound or multi-column key.
Examples
The enrollments table uses a composite key of student_id and course_id.
Column order matters in a composite key because of the index behind it.
Common phrases
composite primary key · composite key of · composite index · multi-column key
More about this term →surrogate key
noun
Meaning
An artificial key with no business meaning, such as an auto-increment number or a UUID, created only to identify each row.
Where you use it
The opposite is a natural key, a real-world value such as a tax number or ISBN. Most modern schemas use surrogate keys because real-world values can change or repeat. A unique constraint on the natural key is still a good idea.
Examples
We switched from national ID numbers to a surrogate key after a customer's ID was entered wrongly.
Every table gets a surrogate key called id, plus unique constraints where needed.
Common phrases
use a surrogate key · surrogate key vs natural key · UUID · artificial key
More about this term →referential integrity
noun
Meaning
The rule that every reference between tables must point to a row that really exists, so there are no orders for missing customers.
Where you use it
Enforced by foreign keys. When it breaks, you get 'orphan' or 'orphaned' rows. It is a common topic in design reviews and data quality reports, and the main argument for keeping foreign keys in the database instead of only in code.
Examples
Without foreign keys we lost referential integrity and found 3,000 orphaned order lines.
The database enforces referential integrity, so you can't delete a category that still has products.
Common phrases
enforce referential integrity · break referential integrity · orphaned rows · data integrity
More about this term →cascade
verb
Meaning
Of a delete or update: to pass automatically from a parent row to all the child rows that point to it through a foreign key.
Where you use it
Set on a foreign key with ON DELETE CASCADE or ON UPDATE CASCADE. Other options are SET NULL, RESTRICT and NO ACTION. Cascading deletes are convenient but dangerous: one DELETE can silently remove thousands of related rows.
Examples
Deleting a customer cascades to their addresses and payment methods.
Be careful: the foreign key is set to cascade, so this will also delete all the invoices.
Common phrases
ON DELETE CASCADE · cascading delete · cascade to child rows · ON UPDATE CASCADE
More about this term →default value
noun
Meaning
The value a column gets automatically when a new row is inserted without a value for that column, such as 0, 'pending' or the current time.
Where you use it
Set with DEFAULT in CREATE TABLE or ALTER TABLE, for example created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP. Defaults are important when adding a NOT NULL column to a table that already has data, because existing rows need a value too.
Examples
Give the status column a default value of 'pending'.
The new column has no default value, so all old rows show NULL.
Common phrases
set a default value · DEFAULT CURRENT_TIMESTAMP · default value of · column default
More about this term →denormalization
noun
Meaning
Deliberately storing some data in more than one place, or combining tables, to make reads faster, while accepting more work to keep the copies correct.
Where you use it
Common in reporting databases, data warehouses and NoSQL designs. Examples include storing an order total instead of calculating it or copying the customer name into the orders table. It should be a conscious decision after normalizing, not an excuse for poor design.
Examples
We accepted some denormalization in the reporting tables to avoid eight-table joins.
Denormalization speeds up the product page, but now the price is stored in two places.
Common phrases
denormalize a table · deliberate denormalization · denormalized data · read performance
More about this term →normal form
noun
Meaning
One of a series of rules for table design (1NF, 2NF, 3NF and higher); each level removes a particular kind of repeated or badly placed data.
Where you use it
In short: 1NF means one value per cell and no repeating groups; 2NF removes columns that depend on only part of a composite key; 3NF removes columns that depend on other non-key columns. Most business schemas aim for third normal form.
Examples
Storing three phone numbers in one column breaks first normal form.
The schema is in third normal form except for the reporting tables.
Common phrases
first normal form · third normal form · 1NF · 3NF · Boyce-Codd normal form
More about this term →cardinality
noun
Meaning
Either the type of relationship between tables (one-to-one, one-to-many, many-to-many) or the number of distinct values in a column.
Where you use it
Context decides the meaning. In data modeling it describes relationships on an ER diagram. In performance work, 'high cardinality' means many distinct values (like email) and 'low cardinality' means few (like status); high-cardinality columns usually make better index candidates.
Examples
An index on the gender column won't help much because its cardinality is very low.
Check the cardinality of each relationship before you draw the foreign keys.
Common phrases
high cardinality · low cardinality · relationship cardinality · cardinality estimate
More about this term →clustered index
noun
Meaning
An index that decides the physical order in which a table's rows are stored, so a table can have only one; the other indexes are non-clustered.
Where you use it
A key term in SQL Server, where the primary key becomes the clustered index by default, and in MySQL InnoDB, where the primary key is always the clustered index. PostgreSQL has no permanent clustered index; its CLUSTER command reorders a table only once.
Examples
A random GUID as the clustered index causes page splits on every insert.
The table has a clustered index on order_date and two non-clustered indexes.
Common phrases
clustered index · non-clustered index · clustered index scan · clustered index key
More about this term →full table scan
noun
Meaning
Reading every row of a table to find the ones a query needs, instead of jumping straight to them through an index.
Where you use it
Execution plans show it as 'Seq Scan' in PostgreSQL, 'type: ALL' in MySQL and 'Table Scan' or 'Clustered Index Scan' in SQL Server. On big tables it is a common cause of slow queries, but on small tables, or when most rows are needed anyway, it can actually be the fastest choice.
Examples
Because the WHERE clause wraps the column in a function, the database does a full table scan.
A full table scan on 200 rows is nothing to worry about.
Common phrases
do a full table scan · avoid a full table scan · sequential scan · table scan
More about this term →slow query
noun
Meaning
A query that takes longer than an agreed limit to finish, slowing down the application and using up database resources.
Where you use it
Databases can record them in a slow query log: MySQL has slow_query_log with long_query_time, and PostgreSQL uses log_min_duration_statement. Teams review the slowest queries regularly and fix them with indexes, rewrites or caching.
Examples
The slow query log shows the same report query taking over 30 seconds every hour.
One slow query held a lock and blocked the whole checkout flow.
Common phrases
slow query log · find slow queries · fix a slow query · top slow queries
More about this term →isolation level
noun
Meaning
A setting that controls how much one transaction can see of other transactions' changes, trading data safety against speed and concurrency.
Where you use it
The SQL standard defines four: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ and SERIALIZABLE. Lower levels allow problems such as dirty reads and phantom reads. PostgreSQL and SQL Server default to READ COMMITTED; MySQL InnoDB defaults to REPEATABLE READ.
Examples
We raised the isolation level to SERIALIZABLE for the seat booking transaction.
What is the default isolation level in MySQL, and why does it matter for this bug?
Common phrases
set the isolation level · transaction isolation level · READ COMMITTED · dirty read · SERIALIZABLE
More about this term →lock
noun
Meaning
A mark the database places on a row, page or table so that other transactions cannot change it, or sometimes read it, until the current transaction ends.
Where you use it
Locks can be row-level or table-level, and shared (for reading) or exclusive (for writing). People say a query 'takes', 'holds', 'waits for' or 'releases' a lock. Long-held locks cause blocking and timeouts, so DBAs monitor lock waits closely.
Examples
The batch job holds a lock on the inventory table for almost a minute.
Use SELECT ... FOR UPDATE to lock the row before you change the balance.
Common phrases
hold a lock · release a lock · row-level lock · lock wait · lock timeout
More about this term →deadlock
noun
Meaning
A situation where two or more transactions each wait for a lock the other holds, so none of them can continue until the database cancels one.
Where you use it
Databases detect deadlocks automatically and roll back one transaction, called the deadlock victim, with an error. Applications should catch this error and retry. Accessing tables and rows in the same order in all code paths reduces deadlocks.
Examples
Two jobs update the same accounts in a different order, which causes a deadlock every few hours.
The transaction was chosen as the deadlock victim and rolled back.
Common phrases
deadlock victim · detect a deadlock · deadlock graph · retry on deadlock
More about this term →trigger
noun
Meaning
A piece of code stored in the database that runs automatically when a certain event, such as an insert, update or delete, happens on a table.
Where you use it
Triggers fire BEFORE or AFTER an event, for example to write audit rows or update a last_modified column. They are powerful but hidden: developers may not know a trigger exists, which makes bugs and slow writes hard to explain. You create, drop, enable and disable triggers.
Examples
A trigger writes every price change to the price_history table.
The insert is slow because of a trigger nobody remembered.
Common phrases
create a trigger · trigger fires · AFTER INSERT trigger · disable a trigger
More about this term →migration
noun
Meaning
A versioned script that changes a database schema in a controlled way, or, more broadly, a project to move data from one database or system to another.
Where you use it
In development, migrations are files managed by tools like Flyway, Liquibase, Alembic or Rails and Django migrations. You write, run, apply and roll back migrations. A 'database migration' project, such as Oracle to PostgreSQL, is a much bigger task.
Examples
Write a migration that adds the column, and another one later that drops the old one.
The deploy failed because a migration timed out on the large events table.
Common phrases
run a migration · write a migration · schema migration · roll back a migration · data migration
More about this term →point-in-time recovery
noun
Meaning
Restoring a database to its exact state at a chosen moment, such as one minute before a bad DELETE, by replaying saved change logs on top of a backup.
Where you use it
Shortened to PITR. It needs a base backup plus continuous log archiving: WAL archives in PostgreSQL, binary logs in MySQL and transaction log backups in SQL Server. Cloud databases usually offer it as a setting with a retention period.
Examples
Thanks to point-in-time recovery, we restored the database to 14:02, just before the bad script ran.
Point-in-time recovery only works if log archiving was enabled beforehand.
Common phrases
point-in-time recovery · PITR window · recover to a point in time · log archiving
More about this term →read replica
noun
Meaning
A copy of a database that receives changes from the main server and is used only for read queries, taking load off the primary.
Where you use it
The term is popular on cloud platforms such as Amazon RDS. Reports and search pages read from replicas while writes go to the primary. Because replication is usually asynchronous, a replica may return slightly old data, which surprises users who just saved something.
Examples
Point the reporting service at the read replica, not the primary.
The user saw old data because the read replica was a few seconds behind.
Common phrases
add a read replica · replica lag · read from the replica · promote a replica
More about this term →failover
noun
Meaning
Switching work from a failed primary database server to a standby or replica so that the application can keep running.
Where you use it
Failover can be automatic or manual. A planned, controlled move is often called a switchover. People 'fail over to the standby' (verb, two words) and test failover regularly. After failover, applications must reconnect, and a few seconds of errors are normal.
Examples
The primary crashed at 3 a.m. and automatic failover completed in 40 seconds.
We run a failover test every quarter to make sure the standby really works.
Common phrases
automatic failover · fail over to the standby · failover test · manual failover
More about this term →privilege
noun
Meaning
A permission that allows a database user or role to do a specific action, such as reading a table, inserting rows or creating new objects.
Where you use it
Privileges are granted and revoked with GRANT and REVOKE. Good practice follows the principle of least privilege: each user gets only what it needs, and applications never connect as a superuser. 'Insufficient privileges' and 'permission denied' are common error messages.
Examples
The app user has no DELETE privilege on the audit tables.
Following the principle of least privilege, the analytics account can only read.
Usage note
A very common spelling mistake is 'priviledge'. There is no 'd': privilege.
Common phrases
grant a privilege · revoke a privilege · least privilege · insufficient privileges · admin privileges
More about this term →grant
verb
Meaning
To give a database user or role permission to do something, using the SQL GRANT statement; REVOKE takes the permission away again.
Where you use it
Typical forms: GRANT SELECT ON orders TO report_user; GRANT ALL PRIVILEGES ON DATABASE shop TO app_admin. In conversation, people 'grant access', 'grant read-only access' or 'get access granted'. GRANT and REVOKE are often grouped as DCL (data control language).
Examples
Can you grant me read-only access to the staging database?
We grant privileges to roles, never directly to individual users.
Common phrases
grant access · grant read-only access · GRANT SELECT ON · grant and revoke
More about this term →connection pool
noun
Meaning
A set of open database connections that an application keeps ready and reuses, instead of opening a new, slow connection for every request.
Where you use it
Built into most frameworks (HikariCP in Java, for example) or run as a separate tool such as PgBouncer. Common problems are 'pool exhausted' errors when all connections are busy and too many pools overloading the database's connection limit.
Examples
Requests are timing out because the connection pool has only ten connections.
Each service has its own connection pool, so together they open 800 connections.
Common phrases
connection pool size · pool exhausted · connection pooling · max connections
More about this term →ORM
abbreviation
Meaning
Object-relational mapping: a library that lets developers work with database rows as objects in their programming language and generates the SQL for them.
Where you use it
Popular examples are Hibernate, Entity Framework, Django ORM, SQLAlchemy, Prisma and ActiveRecord. ORMs save time but can hide inefficient queries; the most famous problem is the 'N+1 query problem'. Developers often mix ORM code with raw SQL for complex reports.
Examples
The ORM is sending one query per order item; that's a classic N+1 problem.
For this report, skip the ORM and write the SQL by hand.
Common phrases
use an ORM · ORM framework · N+1 query problem · ORM-generated SQL
More about this term →CRUD
abbreviation
Meaning
Create, Read, Update, Delete: the four basic operations on stored data, matching the SQL statements INSERT, SELECT, UPDATE and DELETE.
Where you use it
Used as an adjective too: 'a CRUD app', 'CRUD screens', 'CRUD endpoints'. It often describes simple business applications whose main job is managing records. In REST APIs the four operations usually map to POST, GET, PUT or PATCH, and DELETE.
Examples
Most of the admin panel is simple CRUD for products and categories.
The intern built a CRUD API for the customer table in two days.
Common phrases
CRUD operations · CRUD app · CRUD endpoints · basic CRUD
More about this term →sharding
noun
Meaning
Splitting one large dataset across several database servers, each holding a different part of the rows, so the system can grow beyond one machine.
Where you use it
Each part is a shard, and rows are placed by a shard key such as customer_id or region. Choosing a good shard key is critical, because queries across shards and resharding are hard. MongoDB, Cassandra and tools like Vitess and Citus support sharding.
Examples
We started sharding by customer ID when the main database passed four terabytes.
Sharding makes reports across all customers much harder to run.
Usage note
Sharding spreads data across servers; partitioning usually splits a table inside one database. People sometimes use the words loosely.
Common phrases
shard key · sharding strategy · shard the database · cross-shard query
More about this term →partitioning
noun
Meaning
Dividing one large table into smaller pieces, called partitions, by a rule such as date range or region, while it still looks like one table to queries.
Where you use it
Common types are range, list and hash partitioning. Benefits include partition pruning (queries skip irrelevant partitions) and easy removal of old data by dropping a whole partition instead of deleting millions of rows.
Examples
Monthly partitioning lets us drop last year's logs in a second.
Partitioning only helps if your queries filter on the partition key.
Common phrases
range partitioning · partition key · partition pruning · drop a partition
More about this term →high availability
noun
Meaning
Designing a database system so that it keeps working with very little downtime, even if a server, disk or data center fails.
Where you use it
Shortened to HA, as in 'an HA setup' or 'HA cluster'. It is built with replication, standby servers and automatic failover, and often measured in 'nines': 99.99% availability allows under an hour of downtime per year. HA does not replace backups.
Examples
For high availability we run a primary and two standbys in different zones.
The contract requires high availability of 99.95% for the payments database.
Common phrases
high availability cluster · HA setup · design for high availability · 99.99% uptime
More about this term →dump
noun
Meaning
A file that contains a database's structure and data written out as SQL statements or in a special export format, which can be loaded into another database.
Where you use it
Created with tools such as pg_dump and mysqldump; this is a logical backup, as opposed to a physical copy of data files. 'Dump' is also a verb: 'dump the database', 'dump a table to CSV'. Dumps are handy for moving data between versions or environments.
Examples
Send me a dump of the test database without the customer emails.
A full dump of this database takes three hours to restore.
Usage note
In everyday English 'dump' can also mean rubbish or a place for it, and 'take a dump' is a rude slang phrase; in a technical context colleagues understand it correctly, but 'create a dump' is safer in formal writing.
Common phrases
create a dump · database dump · SQL dump · load a dump · pg_dump
More about this term →eventual consistency
noun
Meaning
A guarantee that, if no new changes are made, all copies of the data will become the same after a short time, although reads may briefly return old values.
Where you use it
Common in distributed and NoSQL systems such as Cassandra and DynamoDB, which offer it to stay fast and available. The opposite is strong consistency. In practice, teams design screens so that slightly old data is acceptable, for example like counts.
Examples
Because of eventual consistency, a new comment may take a second to appear in other regions.
Eventual consistency is fine for view counts but not for account balances.
Usage note
False friend: English 'eventual' means 'happening in the end', not 'possible' like Turkish 'eventüel'.
Common phrases
eventual consistency model · eventually consistent · strong consistency · consistency guarantees
More about this term →CAP theorem
noun
Meaning
The idea that when a network failure splits a distributed database, it must choose between consistency (same data everywhere) and availability (always answering).
Where you use it
CAP stands for Consistency, Availability and Partition tolerance. Proposed by Eric Brewer in 2000 and later proven, it is often simplified as 'pick two of three'. It appears in system design interviews and in comparisons of NoSQL databases.
Examples
According to the CAP theorem, during a partition this system prefers availability over consistency.
The interviewer asked me how the CAP theorem applies to our chat service.
Common phrases
CAP theorem · partition tolerance · consistency vs availability · CP system · AP system
More about this term →graph database
noun
Meaning
A NoSQL database that stores data as nodes connected by relationships, making it fast to follow links such as 'friends of friends'.
Where you use it
Neo4j and Amazon Neptune are well-known examples, with query languages such as Cypher and Gremlin. They suit social networks, recommendation engines, fraud detection and network maps, where questions are about connections rather than totals.
Examples
The fraud team uses a graph database to find accounts that share devices and addresses.
A graph database answers 'who else bought this?' much faster than ten SQL joins.
Usage note
Pronounce 'graph' with a short 'a' like 'graf' (US) or a long 'a' like 'grahf' (UK).
Common phrases
graph database · nodes and relationships · graph query · Cypher query
More about this term →collection
noun
Meaning
In a document database such as MongoDB, a group of documents stored together, similar to a table in a relational database.
Where you use it
Unlike a table, a collection does not force every document to have the same fields, although schema validation can be added. You create, drop, query and index collections; MongoDB creates a collection automatically when you first insert into it.
Examples
Each user's settings are stored as one document in the settings collection.
A typo in the collection name created a new, empty collection in production.
Common phrases
create a collection · query a collection · collection name · documents in a collection
More about this term →in-memory database
noun
Meaning
A database that keeps its data mainly in RAM rather than on disk, so reads and writes are extremely fast.
Where you use it
Redis is the best-known example; SAP HANA and in-memory options in SQL Server and Oracle also exist. Because RAM is lost on restart, such systems offer persistence options such as snapshots or append-only logs. They are used for caching, leaderboards, queues and real-time analytics.
Examples
The game stores its live leaderboard in an in-memory database.
Make sure persistence is on, or the in-memory database will lose everything on restart.
Common phrases
in-memory database · in-memory store · persistence options · in-memory cache
More about this term →data warehouse
noun
Meaning
A central database built for reporting and analysis that collects historical data from many systems and is designed for large read queries rather than small updates.
Where you use it
Snowflake, Google BigQuery, Amazon Redshift and Azure Synapse are popular cloud examples. Data is loaded through ETL or ELT pipelines and often modeled as a star schema with fact and dimension tables. It is the typical OLAP system, as opposed to OLTP.
Examples
Sales data is copied to the data warehouse every night for the morning reports.
Run heavy analytics in the data warehouse, not on the production database.
Common phrases
data warehouse · load into the data warehouse · star schema · ETL pipeline · cloud data warehouse
More about this term →OLTP
abbreviation
Meaning
Online transaction processing: database workloads made of many small, fast reads and writes from users, such as placing orders or updating profiles.
Where you use it
Usually contrasted with OLAP (online analytical processing), which means large analytical queries over historical data. Architects say a database is 'built for OLTP' or 'an OLTP system', and warn against running heavy reports on it.
Examples
Our OLTP database handles about 5,000 transactions per second at peak.
Don't mix OLTP and analytics on the same server; the reports slow down checkout.
Common phrases
OLTP system · OLTP workload · OLTP vs OLAP · OLTP database
More about this term →upsert
verb
Meaning
To insert a row if it does not exist yet, or update it if it already does, in a single operation. The word combines 'update' and 'insert'.
Where you use it
Also a noun: 'do an upsert'. Syntax differs: INSERT ... ON CONFLICT DO UPDATE in PostgreSQL and SQLite, INSERT ... ON DUPLICATE KEY UPDATE in MySQL, and MERGE in SQL Server and Oracle. It needs a unique key to decide whether the row exists.
Examples
The sync job upserts products by SKU, so running it twice is safe.
Use an upsert instead of checking first and then inserting; that check can race.
Common phrases
upsert a row · ON CONFLICT DO UPDATE · MERGE statement · upsert by key
More about this term →CTE
abbreviation
Meaning
Common table expression: a named temporary result defined with WITH at the start of a query, which the main query can then use like a table.
Where you use it
CTEs make long queries readable by splitting them into named steps. A recursive CTE (WITH RECURSIVE in PostgreSQL and MySQL) can walk tree data such as category hierarchies or org charts. Developers often say 'pull that out into a CTE'.
Examples
I moved the monthly totals into a CTE so the final SELECT is easier to read.
A recursive CTE returns the whole category tree in one query.
Common phrases
WITH clause · recursive CTE · use a CTE · chain CTEs
More about this term →window function
noun
Meaning
A SQL function that calculates a value across a set of related rows, such as a running total or a rank, while still returning every individual row.
Where you use it
Written with OVER, often with PARTITION BY and ORDER BY: ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY created_at). Common ones are ROW_NUMBER, RANK, LAG, LEAD and SUM OVER. Unlike GROUP BY, rows are not collapsed. Oracle calls them analytic functions.
Examples
Use a window function to get each customer's most recent order.
The running balance column is calculated with a window function over the transactions.
Common phrases
window function · OVER (PARTITION BY ...) · ROW_NUMBER() · running total
More about this term →UNION
noun
Meaning
A SQL operator that stacks the results of two or more SELECT statements into one result, one under the other, removing duplicate rows.
Where you use it
Each SELECT must return the same number of columns with compatible types. UNION ALL keeps duplicates and is faster, because the database does not have to check for them; use it when duplicates are impossible or wanted. A join puts columns side by side; a UNION puts rows on top of each other.
Examples
We UNION the archived orders with the current ones to build the yearly report.
Switch to UNION ALL here; the two tables can't contain the same rows anyway.
Common phrases
UNION ALL · UNION of two queries · UNION vs JOIN · combine results
More about this term →truncate
verb
Meaning
To remove all rows from a table at once with TRUNCATE TABLE, keeping the table and its structure; it is much faster than deleting rows one by one.
Where you use it
TRUNCATE takes no WHERE clause and usually does not fire DELETE triggers. In MySQL and Oracle it commits immediately and cannot be rolled back; in PostgreSQL and SQL Server it can be rolled back inside a transaction. It often resets identity counters too.
Examples
Truncate the staging table before each import so old rows don't get mixed in.
Someone truncated the wrong table, and we had to restore it from last night's backup.
Usage note
DELETE removes chosen rows, TRUNCATE removes all rows but keeps the table, and DROP removes the table itself.
Common phrases
truncate a table · TRUNCATE TABLE · truncate vs delete · truncate and reload
More about this term →DDL
abbreviation
Meaning
Data definition language: the SQL statements that create, change and remove database objects, such as CREATE, ALTER and DROP.
Where you use it
People say 'the DDL for this table' to mean its CREATE script, or 'DDL changes' for schema changes. DDL often needs higher privileges and extra review, and in some databases, such as MySQL and Oracle, it commits the current transaction automatically.
Examples
Can you send me the DDL for the invoices table?
All DDL changes in production need a ticket and DBA review.
Common phrases
DDL statement · DDL changes · generate DDL · DDL script
More about this term →DML
abbreviation
Meaning
Data manipulation language: the SQL statements that work with the data inside tables, mainly INSERT, UPDATE, DELETE and often SELECT.
Where you use it
Some sources list SELECT separately as DQL (data query language). The distinction matters for permissions, auditing and change control: DML changes data, DDL changes structure. MERGE is also counted as DML.
Examples
The audit trigger records every DML operation on the salaries table.
Developers can run DML on staging but not DDL.
Common phrases
DML statement · DML operation · DML vs DDL · DML trigger
More about this term →query tuning
noun
Meaning
The work of making slow queries faster by studying their execution plans and then adding indexes, rewriting SQL or changing database settings.
Where you use it
A core DBA and senior developer skill, also called query optimization or performance tuning. You tune a query, and the usual cycle is: measure, read the plan, change one thing, measure again. Fixing the query is usually better than adding hardware.
Examples
After a week of query tuning, the nightly batch finishes in 20 minutes instead of three hours.
Before we buy a bigger server, let's try some query tuning.
Common phrases
query tuning · tune a query · performance tuning · query optimization
More about this term →role
noun
Meaning
A named set of database privileges that can be given to many users at once, such as read_only or app_writer, to make permission management simpler.
Where you use it
Instead of granting privileges user by user, DBAs grant them to roles and then assign roles to users. In PostgreSQL users and roles are technically the same thing. Phrases include 'create a role', 'assign a role' and 'role-based access control' (RBAC).
Examples
New analysts get the reporting role, which can read but not modify any table.
Create a separate role for the migration tool with DDL privileges.
Common phrases
create a role · assign a role · role-based access control · read-only role
More about this term →Web, Mobile and Interface Development
front end
noun
Meaning
The part of a website or web app that users see and interact with in the browser, built mainly with HTML, CSS and JavaScript.
Where you use it
Used in job titles, team names and planning meetings. Before a noun it is written with a hyphen (front-end developer, front-end code); many companies simply write frontend as one word.
Examples
The API is ready, but the front end still shows the old prices.
We're hiring two front-end developers who know React.
Usage note
Front end (noun), front-end (adjective) and frontend are all common; just be consistent within one document.
Common phrases
front-end developer · front-end code · work on the front end · front-end framework
More about this term →back end
noun
Meaning
The server side of a website or app that users do not see: it stores data, runs business logic and sends the front end what it needs.
Where you use it
Back-end developers work with servers, databases and APIs, using languages and platforms such as PHP, Python, Java, C#, Go and Node.js. As with front end, the hyphen appears before a noun: back-end service, back-end team.
Examples
The page is slow because the back end takes three seconds to answer.
She moved from front-end work to the back-end team last year.
Common phrases
back-end developer · back-end service · on the back end · back-end logic
More about this term →full stack
adjective
Meaning
Covering both the front end and the back end of a web application, from the user interface to the server and the database.
Where you use it
Mostly used in job titles and CVs (full-stack developer, full-stack engineer). It also works as a noun phrase: 'She works across the full stack.'
Examples
As a full-stack developer, I write both the React pages and the Node.js API.
Small startups often look for full-stack engineers who can ship a feature alone.
Common phrases
full-stack developer · full-stack engineer · full-stack framework · work across the full stack
More about this term →browser
noun
Meaning
A program such as Chrome, Firefox, Safari or Edge that downloads web pages and turns their code into the screens people read and use.
Where you use it
Developers test their work in several browsers and talk about browser support, browser bugs and browser extensions. 'In the browser' often means 'on the client side', as opposed to on the server.
Examples
It works in Chrome. Which browser are you using?
Clear your browser cache and reload the page.
This code runs in the browser, so never put secret keys in it.
Common phrases
browser support · browser extension · browser cache · open in a browser · modern browsers
More about this term →HTML
abbreviation
Meaning
HyperText Markup Language: the standard language for describing the structure and content of a web page, such as headings, paragraphs, links and images.
Where you use it
Said letter by letter. Developers write and validate HTML and talk about HTML files, HTML templates and the HTML a framework outputs. People still say HTML5, although the standard is now simply updated as the HTML Living Standard.
Examples
Can you check the HTML? The button is outside the form.
The server returns plain HTML, so the page loads fast even without JavaScript.
Usage note
HTML is a markup language, not a programming language: it describes content but has no logic such as loops or conditions.
Common phrases
write HTML · valid HTML · HTML file · HTML template · HTML element
More about this term →CSS
abbreviation
Meaning
Cascading Style Sheets: the language that controls how HTML looks, including colors, fonts, spacing, layout and animations.
Where you use it
Said letter by letter. You write, add and override CSS, and work with CSS files, CSS frameworks such as Tailwind or Bootstrap, and CSS-in-JS libraries. 'Cascading' refers to the rules that decide which style wins.
Examples
I fixed the alignment with three lines of CSS.
Our CSS is getting messy, so we're moving to Tailwind.
Common phrases
write CSS · override CSS · CSS file · CSS framework · inline CSS
More about this term →JavaScript
noun
Meaning
The programming language that runs in every web browser and makes pages interactive; with Node.js it also runs on servers.
Where you use it
Often shortened to JS in speech and file names (app.js). Developers write, load, bundle and minify JavaScript; 'vanilla JavaScript' means plain JavaScript without a framework.
Examples
The menu doesn't open because a JavaScript error stops the script.
We built the first version in vanilla JavaScript, without any framework.
Usage note
JavaScript has nothing to do with Java except part of its name. Its official standard is called ECMAScript (ES2015, ES2024 and so on).
Common phrases
vanilla JavaScript · JavaScript file · JavaScript error · load JavaScript · disable JavaScript
More about this term →URL
abbreviation
Meaning
Uniform Resource Locator: the full address of a page or file on the web, such as https://example.com/blog?page=2.
Where you use it
Usually said letter by letter. Developers talk about the parts of a URL (protocol, domain, path, query string and fragment) and say things like 'paste the URL' or 'the URL changes when you filter'.
Examples
Can you send me the URL of the page where you see the error?
Keep the search filters in the URL so users can share the results.
Usage note
Some developers pronounce URL as one word, 'earl', but saying it letter by letter is the safer choice in meetings.
Common phrases
paste the URL · URL path · query string · canonical URL · clean URLs
More about this term →server
noun
Meaning
A computer or program that waits for requests from browsers and other clients and sends back pages, files or data.
Where you use it
The word means both the machine and the software running on it. Developers start, restart and deploy to servers, and say things like 'the server is down' or 'it works on my local server'.
Examples
The server is down again, so nobody can log in.
Run npm run dev to start the local server on port 3000.
Common phrases
start the server · web server · local server · server error · the server is down
More about this term →client
noun
Meaning
The side of a connection that asks for data, usually the browser or a mobile app, as opposed to the server that answers.
Where you use it
Common in 'client side', 'client-side code' and 'client-server model'. In agency work 'client' also means the customer who pays for the website, so the context decides the meaning.
Examples
We validate the form on the client and again on the server.
Move this calculation to the client side so the server has less work.
Common phrases
client side · client-side code · client-server model · API client
More about this term →element
noun
Meaning
A single building block of an HTML page, such as a paragraph, image or button, usually written as an opening tag, content and a closing tag.
Where you use it
Used constantly in HTML, CSS and JavaScript: you select, style, hide, create and remove elements. Browser developer tools let you 'inspect element' to see its code and styles.
Examples
Right-click the button and choose Inspect to see which element is on top.
The script creates a new list element for every comment.
Usage note
A tag is the code in angle brackets (<p>); an element is the whole thing, from the opening tag to the closing tag, including its content.
Common phrases
HTML element · inspect element · parent element · child element · create an element
More about this term →tag
noun
Meaning
A piece of HTML code in angle brackets, such as <h1> or </p>, that marks where an element starts or ends.
Where you use it
Developers talk about opening and closing tags, tags without a closing pair such as <img>, and specific ones like the title tag, meta tags and script tags. Outside HTML, 'tag' can also mean a label on a blog post or a version marker in Git.
Examples
You forgot the closing tag, so the whole footer is inside the link.
Add the script tag at the end of the body.
Common phrases
opening tag · closing tag · script tag · self-closing tag · title tag
More about this term →attribute
noun
Meaning
Extra information written inside an opening HTML tag as a name and a value, such as href in a link or alt in an image.
Where you use it
You add, set, remove and read attributes. Common ones are id, class, href, src, alt and type; custom data-* attributes store your own values for JavaScript.
Examples
The image has no alt attribute, so screen readers may read out the file name instead.
Store the product ID in a data attribute on the button.
Usage note
Watch the stress: the noun is AT-tri-bute, the verb (to attribute something to someone) is at-TRI-bute. In CSS the matching word is property, not attribute.
Common phrases
add an attribute · alt attribute · data attribute · attribute value · boolean attribute
More about this term →selector
noun
Meaning
The part of a CSS rule that says which elements the styles apply to, for example p, .button or #header.
Where you use it
Developers write, combine and simplify selectors, and the same selectors are used in JavaScript with querySelector(). Common types are element, class, ID, attribute and pseudo-class selectors.
Examples
This selector is too specific; just use the class name.
The test fails because the selector no longer matches any element.
Common phrases
CSS selector · class selector · ID selector · match a selector · complex selector
More about this term →DOM
abbreviation
Meaning
Document Object Model: the tree of objects a browser builds from a page's HTML, which JavaScript can read and change while the page is open.
Where you use it
Pronounced as one word, 'dom'. Developers update, query and manipulate the DOM; frameworks such as React use a 'virtual DOM' to work out what to change on the page.
Examples
The script runs before the DOM is ready, so it can't find the button.
Avoid changing the DOM directly when you use React.
Common phrases
update the DOM · DOM element · DOM tree · virtual DOM · manipulate the DOM
More about this term →responsive design
noun
Meaning
An approach to building pages so that the same layout adapts to any screen size, from phones and tablets to large monitors.
Where you use it
Built with flexible layouts, relative units, flexible images and media queries. People say a site 'is responsive' or 'isn't responsive', and 'mobile-first' means designing for small screens first.
Examples
The client wants a responsive design that works well on phones too.
With responsive design, we don't need a separate mobile site anymore.
Common phrases
responsive layout · fully responsive · mobile-first design · responsive images
More about this term →accessibility
noun
Meaning
The practice of making websites usable by everyone, including people who are blind or deaf, have limited movement or use assistive technology.
Where you use it
Often written a11y (a, eleven letters, y). Teams run accessibility audits and test against WCAG, the Web Content Accessibility Guidelines. In many countries, accessibility is also a legal requirement for some websites.
Examples
We ran an accessibility audit and found forty issues on the checkout page.
Good accessibility also helps users on slow phones or in bright sunlight.
Common phrases
accessibility audit · web accessibility · improve accessibility · accessibility issues · WCAG compliance
More about this term →web app
noun
Meaning
An application that runs in a web browser instead of being installed on the device, such as online email, maps or a project management tool.
Where you use it
Short for web application. Developers often contrast a web app (interactive, with logins and data) with a website (mostly content). A progressive web app (PWA) can also be installed and can work offline.
Examples
It started as a simple website and grew into a full web app with user accounts.
Our web app works on any device with a modern browser.
Common phrases
build a web app · progressive web app · web app development · single-page web app
More about this term →developer tools
noun
Meaning
Tools built into browsers that let developers inspect HTML and CSS, debug JavaScript, watch network requests and measure performance.
Where you use it
Usually opened with F12 or with right-click and Inspect. People call them dev tools or DevTools and refer to the panels by name: Elements, Console, Network, Sources and Performance.
Examples
Open the developer tools and check the Console for errors.
I used the browser's developer tools to test the page on a small screen.
Common phrases
open the developer tools · browser developer tools · Network tab · inspect an element
More about this term →property
noun
Meaning
In CSS, the name of a style feature you set, such as color, margin or display, which is always paired with a value.
Where you use it
A CSS declaration has the form 'property: value;'. Developers set, change and override properties; custom properties (CSS variables such as --main-color) store reusable values. JavaScript objects have properties too.
Examples
Which CSS property makes the text bold? It's font-weight.
Use a custom property for the brand color so we can change it in one place.
Usage note
In English, HTML elements have attributes and CSS rules have properties. Turkish speakers often mix them up because both can be translated as 'özellik'.
Common phrases
CSS property · custom property · set a property · shorthand property
More about this term →stylesheet
noun
Meaning
A file or block of CSS rules that describes how a page should look; most sites link to one or more external .css files.
Where you use it
You link, load or import a stylesheet, usually with a <link> tag in the head. Browsers also have their own default (user agent) stylesheets, which is why projects often add a reset or normalize file.
Examples
The page looks unstyled because the stylesheet returns a 404.
Load the print stylesheet only when the user prints the page.
Common phrases
link a stylesheet · external stylesheet · global stylesheet · print stylesheet
More about this term →layout
noun
Meaning
The way elements are arranged on a page: where columns, menus, images and text sit and how they move when the screen size changes.
Where you use it
Developers build, fix and test layouts, and say a layout 'breaks', 'shifts' or 'overflows'. The main CSS layout methods are flexbox, grid and positioning.
Examples
The layout breaks on screens narrower than 360 pixels.
We'll use a two-column layout on desktop and a single column on mobile.
Usage note
The noun is one word (layout); the verb is two words: 'lay out the cards in a grid'.
Common phrases
page layout · two-column layout · the layout breaks · layout shift · fix the layout
More about this term →flexbox
noun
Meaning
A CSS layout method for arranging items in a single row or column and controlling how they grow, shrink, align and share space.
Where you use it
Turned on with display: flex. The official name is the Flexible Box Layout Module, but everyone says flexbox. Common properties are justify-content, align-items, gap and flex-wrap.
Examples
Use flexbox to center the icon and the text inside the button.
The cards wrap onto a new line because the flexbox container has flex-wrap set.
Usage note
A common rule of thumb: flexbox for one dimension (a row or a column), grid for two dimensions (rows and columns together).
Common phrases
use flexbox · flexbox layout · flex container · flex item
More about this term →CSS grid
noun
Meaning
A CSS layout system for placing elements in rows and columns at the same time, useful for full page layouts and card galleries.
Where you use it
Turned on with display: grid. Developers define columns and rows (grid-template-columns), use the fr unit and name grid areas. Often just called 'grid': 'Let's do this with grid.'
Examples
With CSS grid, the gallery shows four columns on desktop and two on tablets.
I rebuilt the dashboard with CSS grid and removed half of the wrapper divs.
Common phrases
grid layout · grid template · grid area · fr unit · display: grid
More about this term →media query
noun
Meaning
A CSS rule that applies styles only when a condition is true, such as a minimum screen width, dark mode or printing.
Where you use it
Written with @media, for example @media (min-width: 768px). Media queries are the main tool of responsive design; developers write and adjust them for each breakpoint.
Examples
Add a media query so the sidebar moves under the content on mobile.
This media query switches the colors when the user prefers dark mode.
Common phrases
write a media query · min-width media query · print media query · prefers-color-scheme
More about this term →component
noun
Meaning
A reusable, self-contained piece of a user interface, such as a button, modal or product card, with its own markup, styles and logic.
Where you use it
Central to React, Vue, Angular and Svelte. Developers create, reuse, render and split components, pass data to them as props, and build component libraries or design systems.
Examples
This page is too big; let's split it into smaller components.
The Button component accepts a size prop: small, medium or large.
Common phrases
reusable component · UI component · component library · child component · render a component
More about this term →