熔岩是指火山喷发出来的熔化岩石（即岩浆），或这些岩石冷固之后的形成物。地球和其他一些类地行星的内核是由岩浆组成的。在地球中，使得岩石熔化的热力来自地热能。当火山喷发时，熔岩就会喷洒出来，最初温度可达700～1,200 °C（1,292～2,192 °F）。它比水浓稠十万倍，但也可以蜿蜒流动，由于其触变性和剪切稀化的特性，之后会慢慢冷却凝固变成火成岩。

在溢流喷发时更容易形成熔岩流，而喷发则会形成火山灰和火山碎屑等物质，熔岩流反而不常见。拉丁语系中常用的“lava”一词来自意大利语，源头是拉丁语“labes”，意思是“摔下”。

Lava is the molten rock expelled by a volcano during an eruption. The resulting rock after solidification and cooling is also called lava. The molten rock is formed in the interior of some planets, including Earth, and some of their satellites. The source of the heat that melts the rock within the earth is geothermal energy. When first erupted from a volcanic vent, lava is a liquid at temperatures from 700 to 1,200 °C (1,292 to 2,192 °F).