girlfriend vs adult female

girlfriend and adult female both are nouns.

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

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

Other hypernyms of girlfriend include lover, woman.

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

Other hypernyms of girlfriend include friend, woman.

girlfriend (noun)	adult female (noun)
a girl or young woman with whom a man is romantically involved	an adult female person (as opposed to a man)
any female friend