B1 [C or U] a substance that is used to provide heat or power, usually by being burned: Wood, coal, oil, petrol and gas are all different kinds of fuel. Plutonium is a fuel used to produce nuclear energy. nuclear fuel unleaded fuel The new exhaust system, it is claimed, will lower fuel consumption.FuelsPetroleum products especially when used as fuel › [U] anything that keeps people's ideas or feelings active, or makes them stronger: Reports in today's newspapers have added fuel to the controversy (= made it worse).Causing things to happen