Definition:
Atomicity Consistency Isolation Durability (ACID) is an acronym representing the following properties:
Atomicity. In a transaction involving two or more discrete pieces of information, either all of the pieces are committed or none are.
Consistency. A transaction either creates a new and valid state of data, or, if any failure occurs, returns all data to its state before the transaction was started.
Isolation. A transaction in process and not yet committed must remain isolated from any other transaction.
Durability. Committed data is saved by the system such that, even in the event of a failure and system restart, the data is available in its correct state.
Further Reading:
Book: Patterns of Enterprise Application Architecture by Martin Fowler .