hitman vs hit man

hitman and hit man both are nouns.

Word	Noun	Adjective	Verb	Adverb
hitman	Yes	No	No	No
hit man	Yes	No	No	No

As nouns, hitman and hit man are synonyms defined as:

Other synonyms of hitman include gun, gun for hire, gunman, gunslinger, hired gun, shooter, torpedo, triggerman.

hitman (noun)	hit man (noun)
a professional killer who uses a gun	a professional killer who uses a gun