bachelorette vs adult female

bachelorette and adult female both are nouns.

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

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

Other hypernyms of bachelorette include woman.

bachelorette (noun)	adult female (noun)
a young unmarried woman who lives alone	an adult female person (as opposed to a man)