washroom vs lavatory

washroom and lavatory both are nouns.

Word	Noun	Adjective	Verb	Adverb
washroom	Yes	No	No	No
lavatory	Yes	No	No	No

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

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

washroom (noun)	lavatory (noun)
a lavatory (particularly a lavatory in a public place)	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