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.