alumna vs adult female

alumna and adult female both are nouns.

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

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

alumna: a female person who has received a degree from a school (high school or college or university)
adult female: an adult female person (as opposed to a man)

Other hypernyms of alumna include alum, grad, graduate, woman.

alumna (noun)	adult female (noun)
a female person who has received a degree from a school (high school or college or university)	an adult female person (as opposed to a man)