A fuel is any substance that may be used to produce heat, light, or power. Traditional fuels include dried dung, animal and vegetable oil, wood, peat, and coal, supplemented by the manufactured fuels charcoal, coal gas, coke, and water gas. In the 20th century, petroleum and natural gas came into widespread use.
The term "fuel" has been extended to include chemical and nuclear fuels, although these are not burned. Specialized high-energy fuels such as hydrazine are used in rocket engines. In the case of a liquid-propellant rocket engine, fuel is ordinarily distinguished from oxidizer where these are separate.
The chief property of a fuel is its calorific value – the amount of heat produced by complete combustion of a unit mass or volume of fuel. Also of major importance is the proportion of incombustibles – ash and moisture – and of sulfur and other compounds liable to cause air pollution.