Combustible means capable of catching fire and burning. A combustible material is something that will burn if exposed to enough heat or flame. Wood, paper, gasoline, and dry leaves are all combustible. Many metals and stones are not combustible because they don't burn under normal conditions.

In science class, you might learn which materials are combustible and which aren't. This matters enormously in the real world: architects choose non-combustible materials like concrete and steel for buildings to help prevent fires from spreading. Fire codes require combustible materials like cardboard boxes to be stored safely away from heat sources.

Something highly combustible catches fire very easily, like gasoline fumes or dried kindling. Something barely combustible needs intense heat before it will burn. When firefighters talk about combustibles, they mean anything in a building that could fuel a fire.

The word can also describe a tense situation ready to burst into conflict, like when you might say the atmosphere in a room full of arguing people feels combustible. Just as combustible materials need only a spark to ignite, combustible situations need only a small trigger to grow into something bigger.