HQ vs headquarters

HQ and headquarters both are nouns.

Word	Noun	Adjective	Verb	Adverb
HQ	Yes	No	No	No
headquarters	Yes	No	No	No

As nouns, HQ and headquarters are synonyms defined as:

HQ and headquarters: the military installation from which a commander performs the functions of command

Other synonyms of HQ include military headquarters.

HQ (noun)	headquarters (noun)
the military installation from which a commander performs the functions of command	the military installation from which a commander performs the functions of command
	(usually plural) the office that serves as the administrative center of an enterprise
	(plural) a military unit consisting of a commander and the headquarters staff