water closet vs lav

water closet and lav both are nouns.

Word	Noun	Adjective	Verb	Adverb
water closet	Yes	No	No	No
lav	Yes	No	No	No

As nouns, lav is a hypernym of water closet; that is, lav is a word with a broader meaning than water closet:

Other hypernyms of water closet include bathroom, can, john, lavatory, privy, toilet.

water closet (noun)	lav (noun)
a toilet in Britain	a room or building equipped with one or more toilets