millionairess vs adult female

millionairess and adult female both are nouns.

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

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

Other hypernyms of millionairess include have, rich person, wealthy person, woman.

millionairess (noun)	adult female (noun)
a woman millionaire	an adult female person (as opposed to a man)