fiancee vs adult female

fiancee and adult female both are nouns.

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

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

Other hypernyms of fiancee include betrothed, woman.

fiancee (noun)	adult female (noun)
a woman who is engaged to be married	an adult female person (as opposed to a man)