restroom vs men's room

restroom and men's room both are nouns.

Word	Noun	Adjective	Verb	Adverb
restroom	Yes	No	No	No
men's room	Yes	No	No	No

As nouns, men's room is a hyponym of restroom; that is, men's room is a word with a more specific, narrower meaning than restroom:

Other hyponyms of restroom include ladies' room, powder room, latrine, men's.

restroom (noun)	men's room (noun)
a toilet that is available to the public	a public toilet for men