What are ACID properties in databases?

ACID is the set of guarantees that make database transactions reliable.

Atomicity — a transaction is all-or-nothing: either every operation in it succeeds, or none do, with no partial state left behind. Consistency — a transaction moves the database from one valid state to another, respecting all rules and constraints. Isolation — concurrent transactions do not interfere; the result is as if they ran one after another, controlled by isolation levels that trade strictness for performance. Durability — once a transaction commits, its effects survive crashes and power loss, because they are persisted to non-volatile storage.

Together they let you trust operations like transferring money between accounts: the debit and credit happen atomically, never leaving money missing. In an interview, contrast ACID with the BASE model common in NoSQL systems, which relaxes consistency for availability and scale — a useful bridge into the CAP theorem.