flak vs antiaircraft

flak and antiaircraft both are nouns.

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

As nouns, flak and antiaircraft are synonyms defined as:

Other synonyms of flak include ack-ack, ack-ack gun, antiaircraft gun, flack, pom-pom.

flak (noun)	antiaircraft (noun)
artillery designed to shoot upward at airplanes	artillery designed to shoot upward at airplanes
intense adverse criticism
a slick spokesperson who can turn any criticism to the advantage of their employer

flak (adjective)	antiaircraft (adjective)
	designed for defense from a surface position against air attack