A component of ACID; Consistency is a property of a transaction over a database such that the effect of the completed transaction is to produce a new state within the database that is valid and rational. If the transaction is unable for any reason (such as a power failure or system crash) to produce a rational consistent effect on the database, the database must be left in the state it was in before the transaction began.

This is a bit of a simplification, since other transactions may have been occurring concurrently, and because the concept of a rational state is not fully defined here.