Fossil fuel is a general term for buried combustible geologic deposits of organic materials, formed from decayed plants and animals that have been converted to crude oil, coal, natural gas, or heavy oils by exposure to heat and pressure in the earth's crust over hundreds of millions of years.

Generally, "fossil fuels" means coal, oil, and/or natural gas. The term is often used in contrast to other sources of energy like solar, hydro, geothermic, or wind energy.

The word "fossil" come from a Latin word meaning "to dig." A fossil is a plant or animal that has stayed buried deep in the earth for millions of years, and then is dug up. When we think of "a fossil" the first thing we think of is dinosaur bones. However, coal is a fossil. It is the preserved, compressed, transformed plant material like peat. Petroleum is a fossil. It is preserved, compressed, transformed algae and tiny animals, zooplankton. Natural gas is a fossil; it occurs within petroleum deposits. Coal, oil (petroleum), and natural gas are grouped together as "fossil fuels." They share these characteristics: a) They are concentrated sources of energy. Their discovery made the industrial revolution and created the modern world as we know it. b) They need to be extracted from deep in the ground. The extraction process does environmental harm. For example, open pit coal mines create colossal scars on the the landscape. Oil extraction by "fracking" may pollute groundwater. c) They are in limited supply. They were formed very slowly, over millions of years, and we are using them up in just hundreds of years. They once seemed to be unlimited, but now we need to plan for the certainty that will not last forever.