A short, informal British word for a toilet, or for the little room the toilet is in. Someone might say “I’m just going to the lav.” It is friendly and a bit old-fashioned. Younger British speakers more often say loo, and many people just say toilet. Americans do not use lav this way at all; they say bathroom or restroom. You would not write it in a formal letter or on a public sign — those still say lavatory or toilet. It is spoken, casual, and British.