mistress has 5 distinct synonyms spread across 2 of its 3 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. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
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.
an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
Synonyms, antonyms and the broader and narrower relations for mistress come from Princeton University’s WordNet. Full sources and licences.
a woman schoolteacher (especially one regarded as strict)
a woman master who directs the work of others