fuel

Any source of heat or energy, embracing the entire range of materials that burn in air (combustibles). A fuel is a substance that gives out energy when it burns. A nuclear fuel is any material that produces energy by nuclear fission in a nuclear reactor. Fossil fuels are formed from the fossilized remains of plants and animals.

Crude oil (unrefined petroleum) is purified at an oil refinery by fractional distillation into fuels such as gasoline and kerosene. The burning of fossil fuels for energy production contributes to environmental problems such as acid rain and the greenhouse effect.