|An intensely hot pool of molten rock, emitting clouds of sulphurous smoke.|
Lava is a type of terrain found in the Dungeon (usually on deeper floors) and many of its branches. If you are flying above it you get a status effect 'LAVA' coloured red as a warning. It is impossible to cross lava for most player species, unless you can fly. Attempting to move onto lava is not normally allowed.
Having one's means of flight expire while over lava results in an emergency flight mode of sorts that causes rather heavy draining per turn spent in lava.
You can use confusion to your advantage (e.g. Confusion, Mephitic Cloud) by targeting monsters near lava. They may attempt to cross it and fall in, killing them instantly. Keep in mind that this will also destroy their equipment in the process.
Prior to 0.17, you could accidentally step into lava while confused.