- The definition of fossil fuel is an organic substance from the geologic period which is found underground and is used as a source of energy.
An example of a fossil fuel is coal.
Gasoline is a fossil fuel.
an organic substance, as coal, petroleum, etc., found underground in deposits formed in a previous geologic period and used as a source of energy
A hydrocarbon-based fuel, such as petroleum, coal, or natural gas, derived from living matter of a previous geologic time.