empress vs adult female

empress and adult female both are nouns.

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

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

Other hypernyms of empress include emperor, woman.

empress (noun)	adult female (noun)
a woman emperor or the wife of an emperor	an adult female person (as opposed to a man)