The Free On-line Dictionary of Computing (30 December 2018):

back-end

    Any software performing either the final stage
   in a process, or a task not apparent to the user.  A common
   usage is in a compiler.  A compiler's back-end generates
   machine language and performs optimisations specific to the
   machine's architecture.

   The term can also be used in the context of network
   applications.  E.g. "The back-end of the system handles
   socket protocols".

   Contrast front end.

   (1996-04-09)