background: n.,adj.,vt. To do a task `in background' is to do it whenever {foreground} matters are not claiming your undivided attention,

:background: n.,adj.,vt. To do a task `in background' is to do
it whenever {foreground} matters are not claiming your undivided
attention, and `to background' something means to relegate it to
a lower priority. "For now, we'll just print a list of nodes and
links; I'm working on the graph-printing problem in background."
Note that this implies ongoing activity but at a reduced level or
in spare time, in contrast to mainstream `back burner' (which
connotes benign neglect until some future resumption of activity).
Some people prefer to use the term for processing that they have
queued up for their unconscious minds (a tack that one can often
fruitfully take upon encountering an obstacle in creative work).
Compare {amp off}, {slopsucker}.

Technically, a task running in background is detached from the
terminal where it was started (and often running at a lower
priority); oppose {foreground}. Nowadays this term is primarily
associated with {{UNIX}}, but it appears to have been first used
in this sense on OS/360.
-- The AI Hackers Dictionary