Lau*sanne a town in southwestern Switzerland, on the north shore of Lake Geneva; pop. 123,000. Lau*sit*zer Neis*se German name for NEISSE (sense 1). Linked entries: NEISSE # lav n. INFORMAL a lavatory. early 20th cent.: abbreviation. la*va n. hot molten or semifluid rock erupted from a volcano or fissure, or solid rock resulting from cooling of this. mid 18th cent.: from Italian (Neapolitan dialect), denoting the lava stream from Vesuvius, but originally denoting a stream caused by sudden rain, from lavare 'to wash', from Latin.