mistress vs kept woman

mistress and kept woman both are nouns.

Word	Noun	Adjective	Verb	Adverb
mistress	Yes	No	No	No
kept woman	Yes	No	No	No

As nouns, mistress and kept woman are synonyms defined as:

mistress and kept woman: an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man

Other synonyms of mistress include fancy woman.

mistress (noun)	kept woman (noun)
a woman master who directs the work of others	an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
a woman schoolteacher (especially one regarded as strict)