[from Gk. atomos, indivisible]
1. Indivisible; cannot be split up. 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 esp. to convey that an operation cannot be screwed up by interrupts. “This routine locks the file and increments the file's semaphore atomically.”
2. [primarily techspeak] Guaranteed to complete successfully or not at all, usu. refers to database transactions. If an error prevents a partially-performed transaction from proceeding to completion, it must be “backed out”, as the database must not be left in an inconsistent state.
Computer usage, in either of the above senses, has none of the connotations that ‘atomic’ has in mainstream English (i.e. of particles of matter, nuclear explosions etc.).