Fuel traditionally is a substance that may be burned to produce energy (heat, light, or power). In more recent times fuels can also be nuclear, and these do not require burning to produce energy. The chief property of fuel is its caloric value - the amount of heat produced by complete combustion of a unit mass of its volume.
Many fuels also have a number of incombustibles, such as ash, that could cause air pollution and have unhealthy or even toxic qualities from fuel production.
The New American Desk Encyclopedia, Penguin Group, 1989