viscountess vs adult female

viscountess and adult female both are nouns.

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

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

Other hypernyms of viscountess include peer, woman.

viscountess (noun)	adult female (noun)
a noblewoman holding the rank of viscount in her own right	an adult female person (as opposed to a man)
a wife or widow of a viscount