taskmistress vs adult female

taskmistress and adult female both are nouns.

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

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

Other hypernyms of taskmistress include taskmaster, woman.

taskmistress (noun)	adult female (noun)
a woman taskmaster	an adult female person (as opposed to a man)