A transaction is an action or series of actions which transform the system from one consistent state to another. Atomicity - Refers to the all or nothing property of the transaction. Either the entire transaction completes or none of it does. If the transaction fails to complete successfully, operations performed up to that point are rolled back. The transaction must transform the system from one consistent state to another consistent state.
Consistency is defined by the business rules of the system and enforced by the application.
Although multiple concurrent transactions may occur, during the course of all transactions, it must appear to each transaction that it is the only transaction occurring. Durability - The effects of a committed transaction must remain permanent. In the example of Mary making her VISA payment from her checking account, all of the ACID properties must apply. Assuming that the information about her checking account and the information in her VISA account are in two separate databases , a coordinated transaction must take place. This is usually handled via two phase commits.