To combust means to catch fire and burn. When something combusts, it reacts with oxygen in the air and releases heat and light. A piece of paper combusts when you hold a match to it, and gasoline combusts inside a car's engine to power the vehicle.

The word often describes sudden or rapid burning. Scientists talk about the combustion of fuel in rockets, which creates the force that launches them into space. A pile of oily rags left in a hot garage might spontaneously combust, meaning it catches fire on its own without anyone lighting it, because the oils generate heat as they break down chemically.

People sometimes use the word playfully to exaggerate strong reactions. You might say your older brother looked ready to combust with anger when he discovered someone had eaten his leftover pizza. Or a teacher might joke that her brain will combust from trying to solve a particularly tricky math problem. In these cases, the person isn't actually catching fire, but they're so worked up or mentally exhausted that they feel like they might burst into flames.