criticism vs flak

criticism and flak both are nouns.

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

As nouns, flak is a hyponym of criticism; that is, flak is a word with a more specific, narrower meaning than criticism:

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