What is ACID?

ACID is a set of properties that guarantee the reliability of database transactions:

1. Atomicity, a transaction either runs fully or not at all. If an error occurs midway, the system rolls back every change.
2. Consistency, after a transaction runs, the data stays in a valid state that satisfies the database's rules and constraints.
3. Isolation, concurrent transactions don't interfere with each other: each one behaves as if it were running alone.
4. Durability, once a transaction completes successfully, its changes survive even a system failure.

These properties make transactions predictable and protect data from loss and conflicts.