ACID - Atomicity, Consistency, Isolation, Durability

ACID transactions are a set of features that describe the transactions that a good relational database will support
A transaction is an interaction with a database, typically read or write operations.

Atomicity

Requires that when a single transaction comprises of more than one operation, then the database must guarantee that if one operation fails the entire transaction (all operations) also fail.
It's "all or nothing".
That way if the transaction succeeds, then on completion you know that all the sub-operations completed successfully, and if an operation fails, then you know that all the operations that went with it failed.
For example
- if a single transaction involved reading from two tables and writing to three, then if any one of those individual operations fails the entire transaction fails.
- This means that none of those individual operations should complete. You would not want even 1 out of the 3 write transactions to work - that would "dirty" the data in your databases!

Requires that each transaction in a database is valid according to the database's defined rules, and when the database changes state (some information has changed), such change is valid and does not corrupt the data.
Each transaction moves the database from one valid state to another valid state.
Consistency can be thought of as the following:
- every "read" operation receives the most recent "write" operation results.

means that you can "concurrently" (at the same time) run multiple transactions on a database, but the database will end up with a state that looks as though each operation had been run serially ( in a sequence, like a queue of operations).

is the promise that once the data is stored in the database, it will remain so. It will be "persistent" - stored on disk and not in "memory".