concubine vs kept woman

concubine and kept woman both are nouns.

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

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

concubine: a woman who cohabits with an important man
kept woman: an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man

Other hypernyms of concubine include fancy woman, mistress.

concubine (noun)	kept woman (noun)
a woman who cohabits with an important man	an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man