background has 11 distinct synonyms spread across 7 of its 9 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is foreground.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a person's social heritage: previous experience or training
“he is a lawyer with a sports background”
information that is essential to understanding a situation or problem
“the embassy filled him in on the background of the incident”
extraneous signals that can be confused with the phenomenon to be observed or measured
“they got a bad connection and could hardly hear one another over the background signals”
relatively unimportant or inconspicuous accompanying situation
“when the rain came he could hear the sound of thunder in the background”
the state of the environment in which a situation exists
“you can't do that in a university setting”
(computer science) the area of the screen in graphical user interfaces against which icons and windows appear
understate the importance or quality of
“he played down his royal ancestry”
Synonyms, antonyms and the broader and narrower relations for background come from Princeton University’s WordNet. Full sources and licences.