Software Architect's Handbook

Atomicity

A transaction must be an atomic unit of work, meaning that either all of its data modifications are performed or none at all. This provides reliability because if there is failure in the middle of a transaction, none of the changes in that transaction will be committed. For example, in a financial transaction, you may insert one record to represent the credit part of the transaction and another to represent the debit part of the transaction. You don't want one of those inserts to take place without the other, so you place them both as part of one transaction. Either they will both be committed or neither of them will be committed.