It is a set of properties for database transactions.
Atomicity requires that each transaction be "all or nothing": if one part of the transaction fails, then the entire transaction fails, and the database state is left unchanged.
The consistency property ensures that any transaction will bring the database from one valid state to another.
The isolation property ensures that the concurrent execution of transactions results in a system state that would be obtained if transactions were executed serially, i.e., one after the other.
The durability property ensures that once a transaction has been committed, it will remain so, even in the event of power loss, crashes, or errors.