heiress vs adult female

heiress and adult female both are nouns.

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

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

Other hypernyms of heiress include heir, heritor, inheritor, woman.

heiress (noun)	adult female (noun)
a female heir	an adult female person (as opposed to a man)