Fossil fuels are a type of energy source that is formed from the remains of plants and animals that lived millions of years ago. The four solutions to the crossword for the term fossil fuel are GAS (3 letters), PETROLEUM (9 letters), COAL (4 letters), OIL (3 letters) and PEAT (4 letters). GAS is a combustible mixture of hydrocarbons, primarily methane, used as fuel. PETROLEUM is a flammable liquid consisting of a complex mixture of hydrocarbons obtained from rocks. COAL is a black or dark brown combustible sedimentary rock composed mainly of carbon and hydrocarbons. OIL is a thick, flammable, yellow-to-black mixture of hydrocarbons that occurs naturally in certain rock formations and can be refined to produce gasoline, kerosene, and other products. PEAT is an accumulation of partially decayed vegetation matter found in bogs and fens.