fossil fuel

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A hydrocarbon-based fuel, such as petroleum, coal, or natural gas, derived from living matter of a previous geologic time.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun any fuel derived from hydrocarbon deposits such as coal, petroleum, natural gas and, to some extent, peat; these fuels are irreplaceable, and their burning generates the greenhouse gas carbon dioxide

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content