antiaircraft gun vs flak

antiaircraft gun and flak both are nouns.

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

As nouns, antiaircraft gun and flak are synonyms defined as:

antiaircraft gun and flak: artillery designed to shoot upward at airplanes

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

antiaircraft gun (noun)	flak (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