Lava is magma once it has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or undersea, usually at temperatures from 800 to 1,200 °C (1,470 to 2,190 °F). The volcanic rock resulting from subsequent cooling is also often called lava.
A lava flow is an outpouring of lava created during an effusive eruption. (On the other hand, an explosive eruption produces a mixture of volcanic ash and other fragments called tephra, rather than lava flows.) Although lava can be up to 100,000 times more viscous than water, with a viscosity roughly similar to ketchup, lava can flow great distances before cooling and solidifying because lava exposed to air quickly develops a solid crust that insulates the remaining liquid lava, helping keep it hot and inviscid enough to continue flowing.
The word lava comes from Italian and is probably derived from the Latin word labes which means a fall or slide. The first use in connection with extrusion of magma from below the surface was apparently in a short account written by Francesco Serao on the eruption of Vesuvius in 1737. Serao described "a flow of fiery lava" as an analogy to the flow of water and mud down the flanks of the volcano (a lahar) following heavy rain.