flak vs criticism

flak and criticism both are nouns.

Word	Noun	Adjective	Verb	Adverb
flak	Yes	No	No	No
criticism	Yes	No	No	No

As nouns, criticism is a hypernym of flak; that is, criticism is a word with a broader meaning than flak:

Other hypernyms of flak include unfavorable judgment.

flak (noun)	criticism (noun)
artillery designed to shoot upward at airplanes	a serious examination and judgment of something
intense adverse criticism	disapproval expressed by pointing out faults or shortcomings
a slick spokesperson who can turn any criticism to the advantage of their employer	a written evaluation of a work of literature