kept woman vs mistress

kept woman and mistress both are nouns.

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

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

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

Other synonyms of kept woman include fancy woman.

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