ACID

ACID is the set of guarantees a database makes about transactions, groups of changes that should be treated as one unit, like moving money between two accounts. Atomicity means all of the changes happen or none do, so you never debit one account without crediting the other. Consistency means each transaction takes the data from one valid state to another, respecting rules like “balances can’t go negative”. Isolation means transactions running at the same time don’t see each other’s half-finished work. Durability means once the database says “committed”, the change survives a crash or power cut. Relational databases are built around these promises.