flak vs antiaircraft gun

flak and antiaircraft gun both are nouns.

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

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

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

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

flak (noun)	antiaircraft gun (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