For example, an instruction may be said to do several things "atomically", i.e. all the things are done immediately, and there is no chance of the instruction being half-completed or of another being interspersed. Used especially to convey that an operation cannot be interrupted.
An atomic database transaction is one which is guaranteed to complete successfully or not at all. If an error prevents a partially-performed transaction from proceeding to completion, it must be "backed out" to prevent the database being left in an inconsistent state.