kept woman vs concubine

kept woman and concubine both are nouns.

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

As nouns, concubine is a hyponym of kept woman; that is, concubine is a word with a more specific, narrower meaning than kept woman:

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

Other hyponyms of kept woman include courtesan, doxy, paramour.

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