kept woman vs adult female

kept woman and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
kept woman	Yes	No	No	No
adult female	Yes	No	No	No

As nouns, adult female is a hypernym of kept woman; that is, adult female is a word with a broader meaning than kept woman:

kept woman: an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
adult female: an adult female person (as opposed to a man)

Other hypernyms of kept woman include lover, woman.

kept woman (noun)	adult female (noun)
an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man	an adult female person (as opposed to a man)