Oil is a liquid fossil fuel, is formed from layers of buried plants and animals that have been subjected to geologic heat and pressure over a long period of time. The energy that the plants and animals originally obtained from the sun is stored in the Oil in the form of carbon.

In addition to carbon, Oil contains elements such as nitrogen, sulfur, mercury, lead, and arsenic. Oil is a nonrenewable resource because it cannot be replenished on a human time frame.

Other definition:

Oil is a black, sticky substance used to produce Fuel (petroleum) and materials (plastics).

See crude Oil, petroleum, fossil fuel, hydrocarbons.