1. The molten, fluid rock that issues from a volcano or volcanic vent: Geologists make a difference in meanings of magma, or molten rock underground, and lava which is molten rock on the surface.
2. The rock formed by lava when it solidifies, occurring in many varieties differing greatly in structure and constitution: Lava was appropriately named by people living near Mount Vesuvius which is the only active volcano on the European mainland and it has erupted frequently since Pompeii and Herculaneum were buried by it in A.D. 79.
3. Etymology: first recorded about 1750, from Italian (Neapolitan or Calabrian dialect) lava "torrent, stream", traditionally from Latin lavare "to wash". Originally applied in Italian to flash flood rivulets after downpours, then to streams of molten rock from Vesuvius. Alternate etymology is from Latin labes "a fall" from labi "to fall".

The Neapolitans who lived near Mount Vesuvius took the Italian word lava, meaning "a stream caused suddenly by rain," and applied it to the streams of molten rock coming down the sides of Vesuvius and then the term was then taken into Standard Italian, where it came to mean the "rock" in both its molten and its solidified states.

The Italian word in all its senses was borrowed into English around the middle of the 18th century (1750 being the earliest date on record).