WordNet (r) 3.0 (2006):

front end
    n 1: the side that is forward or prominent [syn: front, front
         end, forepart] [ant: back end, backside, rear]

The Jargon File (version 4.4.7, 29 Dec 2003):

front end
 n.

    1. An intermediary computer that does set-up and filtering for another
    (usually more powerful but less friendly) machine (a back end).

    2. What you're talking to when you have a conversation with someone who is
    making replies without paying attention. ?Look at the dancing elephants!? ?
    Uh-huh.? ?Do you know what I just said?? ?Sorry, you were talking to the
    front end.?

    3. Software that provides an interface to another program ?behind? it,
    which may not be as user-friendly. Probably from analogy with hardware
    front-ends (see sense 1) that interfaced with mainframes.


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

front end

   1. An intermediary computer that does set-up and filtering for
   another (usually more powerful but less friendly) machine (a
   "back end").

   2. Software that provides an interface to another program
   "behind" it, which may not be as user-friendly.  Probably
   from analogy with hardware front-ends that interfaced with
   mainframes.

   [Jargon File]