What is ACID? Describe each property.

ACID is a set of properties that guarantee reliable transactions in databases:

• Atomicity — a transaction is completed in full or not at all. If something goes wrong, all changes are rolled back.
• Consistency — after a transaction, the database remains in a correct state, satisfying all constraints and rules.
• Isolation — parallel transactions do not affect each other, as if they were executed sequentially.
• Durability — after a transaction is committed, its changes are saved even in case of system failures.

Example: when transferring money between accounts, atomicity guarantees that the debit and credit happen together or not at all.