ACID

ACID stands for Atomicity, Consistency, Isolation, and Durability — the four properties that make database transactions reliable even during errors, crashes, or concurrent access.

Atomicity: a transaction fully completes or has no effect. Consistency: every transaction leaves the database in a valid state, honoring all constraints. Isolation: concurrent transactions don't see each other's partial work. Durability: once committed, changes survive a crash.

Relational databases provide ACID guarantees through mechanisms like write-ahead logging, locking, and MVCC.