The ACID test ensures that data has integrity or reliablility. It stands for Atomicity, Consistency, Isolation and Durability.
Atomicity - all steps of the transaction must be completed as a group, otherwise the entire transaction is abandoned.
Consistency - the transaction successfully transforms both the system and the database at the same time, ie. withdrawing money updates your balance in the database and the system gives you the money in your hand at the same time.
Isolation - Concurrent transaction steps are independent from one another and do not interfere with each other's database figures.
Durability - when the changes made by a transaction become permanent after it is completed.
If data pases all of these, it is considered to have integrity, because it ensures the transaction is never complete, inconsistent, mixed up or temporary.
Hope that helps. If you have the Heinemann text book by G.K. Powers its on page 142, but i'm guessing you don't because you said it doesn't cover it. Peaceout, Dale