lesbian vs adult female

lesbian and adult female both are nouns.

Word	Noun	Adjective	Verb	Adverb
lesbian	Yes	Yes	No	No
adult female	Yes	No	No	No

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

Other hypernyms of lesbian include gay, homo, homophile, homosexual, woman.

lesbian (noun)	adult female (noun)
a female homosexual	an adult female person (as opposed to a man)

lesbian (adjective)	adult female (adjective)
of or relating to or characterized by homosexual relations between woman