A Fuel is a substance used to release energy. Archeologic evidence indicates pre-humans learned to release energy, from wood, for cooking, in the distant past.[1]
As human technology developed, fats and oils from plants and animals, were added as fuels. Fuels started to be used for industrial purposes - not just for food preparation.[1] Humans learned to process wood into charcoal, and fats and oils into waxes, which had advantages over purely natural fuels, in portability, safety, and maximum heat production.
Fossil fuels - coal and peat, natural gas and petroleum were harnessed as fuels.[2]
And, when atomic energy was harnessed, the isotopes that drove the energy producing reactions in nuclear reactors and atomic bombs were also referred to as fuels.