lav vs lavatory

lav and lavatory both are nouns.

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

As nouns, lav and lavatory are synonyms defined as:

Other synonyms of lav include bathroom, can, john, privy, toilet.

lav (noun)	lavatory (noun)
a room or building equipped with one or more toilets	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