ACID
In plain terms
A set of guarantees that keep database transactions reliable.
Going deeper
ACID stands for Atomicity, Consistency, Isolation, Durability: a transaction fully completes or fully rolls back, leaves data valid, doesn't interfere with others, and survives crashes once committed. It's the backbone of trustworthy financial and record systems.
Under the hood
Isolation levels (read committed, repeatable read, serializable) trade consistency for concurrency; distributed transactions add cost, which is why many systems relax to BASE.
Examples
Bank transfers, order checkout, PostgreSQL transactions, BEGIN/COMMIT/ROLLBACK.
Think of it like...
Like a careful bank teller: a transfer either moves the full amount or nothing, the books always balance, and once stamped it's permanent.