(1) Any fuel formed from hydrocarbon deposits (e.g. coal, petroleum or oil, natural gas, etc.) or fossilized remains of organism (hence the name).
(2) A solid, liquid or gaseous fuels formed after millions of years by chemical and physical changes in plant and animal residues under high temperature and pressure.
Fossil fuels are important as they can be burned to provide heat. However fossil fuel resources are irreplaceable and limited in supply. That is why there are alternative energy sources being considered. Besides, fossil fuels are known to contribute to greenhouse effect.
Word origin: fossil + fuel
Synonym: mineral fuel
See also: greenhouse gas