ACID defines the guarantees that database transactions provide:
Atomicity: All operations in a transaction succeed or all fail. No partial updates.
Consistency: Database moves from one valid state to another. Constraints are enforced.
Isolation: Concurrent transactions do not interfere with each other.
Durability: Committed data survives crashes.
Relaxing any property has trade-offs. NoSQL databases often sacrifice some ACID guarantees for performance or availability.