water closet vs lavatory

water closet and lavatory both are nouns.

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

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

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

water closet (noun)	lavatory (noun)
a toilet in Britain	a room or building equipped with one or more toilets
	a bathroom sink that is permanently installed and connected to a water supply and drainpipe; where you can wash your hands and face
	a toilet that is cleaned of waste by the flow of water through it