atomic

atomic adj.  [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.).