What is the definition of a fossil fuel?

Fossil fuel is a term that describes fuels made from buried combustible geologic deposits of organic materials
“Fossil fuel means the buried combustible geologic deposits of organic and natural 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 for many years”-ScienceDaily. Fossil fuels are usually found under the Earth.
The image below shows how oil and gas deposits below the ocean floor are formed and exploited:
Fossil fuels take a very long time to form (usually millions of years). Because of this time scale, they are not considered renewable sources of energy.
Fossil fuels are currently the main source of energy for much of the world:
To learn more about fossil fuels, check out this link.