Combustion, the chemical combination of two substances accompanied by the production of light and heat. As commonly used, the term applies to the combination of a substance with oxygen causing fairly rapid oxidation, or fire. Extremely rapid combustion, in which the fuel and oxygen are united instantaneously, is called an explosion. Flame is a product of combustion.

Combustible substances used to produce heat or power by burning are called fuels. Nearly all solid, liquid, and gaseous fuels are made up of compounds of carbon, frequently together with hydrogen. The specially designed place in which fuels are burned is called a combustion chamber. The furnace is a typical combustion chamber. All internal combustion engines have combustion chambers—the space in each cylinder where the air-fuel mixture is compressed, then ignited.

Spontaneous combustion occurs when highly combustible substances such as oily rags or improperly cured hay are left in a poorly ventilated place. These substances begin to unite chemically with the oxygen in the air (that is, they begin to oxidize) and form heat. Lack of air circulation permits the heat to accumulate until the temperature reaches the kindling point, or ignition temperature—the temperature at which the substance will burst into flame.

In a process called combustion synthesis, the self-propagating heat waves produced by combustion are used to create new materials. Materials created by this process are used in a variety of products, including ball bearings, abrasives, and high-temperature superconductors.