When referring to transactions, the acronym ACID defines the following properties :
Atomicity – This is where a group of operations on a set of data is carried out as as though they were a single operation in an all or none fashion.
Consistency – After the transaction is processed the data will reflect the fact that all of the operations were carried out or none of them were carried out if an error occurred.
Isolation – One transaction will not impact on any other transaction. In other words, any data that is part of one transaction can not be modified by another transaction until the first transaction has completed.
Durability – Once a transaction has been completed the results will be permanent. Generally this is achieved by writing the results to disk. Should a transaction be partially processed when a system error occurs the system must recover and leave the data in a consistent state.