mistress vs adult female

mistress and adult female both are nouns.

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

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

Other hypernyms of mistress include employer, woman.

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

mistress: an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
adult female: an adult female person (as opposed to a man)

Other hypernyms of mistress include lover, woman.

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

Other hypernyms of mistress include school teacher, schoolteacher, woman.

mistress (noun)	adult female (noun)
a woman master who directs the work of others	an adult female person (as opposed to a man)
an adulterous woman; a woman who has an ongoing extramarital sexual relationship with a man
a woman schoolteacher (especially one regarded as strict)