A Lava Pit is a fairly rare thing to encounter in real life but is a common day to day feature of video game landscapes the world over.
Lava pits generally take the form of an abyss of lava or some other red hot molten substance that usually resides in a trench or hole in the floor, buy may be as large as a river or lake.
Depending on the whim of some sadistic computer programmers, the exact physical properties of lava pits may be hard to determine in advance, although they are universally unpleasant, the exact degree of unpleasantness may be as small as minor decrease in hit points, or as serious as instant combustion.
Lava pits can be found in Joust, Quake, PlatMan, Super Mario Brothers, and any number of other video games. Lava Pits are also fairly closely related to Acid Pits which have fairly similar properties.