What is a Fuel?

Fuel is any material that is capable of releasing energy when its chemical or physical structure is changed or converted. Fuel releases its energy either through chemical means, such as burning, or nuclear means, such as nuclear fission or nuclear fusion. An important property of a useful fuel is that its energy can be stored to be released only when needed, and that the release is controlled in such a way that the energy can be harnessed to produce work.