The definition of lava is hot, molten rock.(noun)
Hot molten rock that spews from a volcano is an example of lava.
See lava in Webster's New World College Dictionary
Origin: It < dial. (Neapolitan) lave < L labes, a fall, subsidence < labi, to slide < IE base *lāb-, to hang down > lap, lip
See lava in American Heritage Dictionary 4
Origin: , perhaps from Latin lābēs, fall
Origin: , from Latin lābī, to fall. Word History: Lava was appropriately named by people living near Mount Vesuvius. The only active volcano on the European mainland, Vesuvius has erupted frequently since Pompeii and Herculaneum were buried by it in A.D. 79. The Neapolitans who lived in the vicinity 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. 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 of record).
Learn more about lava