The term CONTRAIL SOURCE can be solved by the three letter word JET. A JET is an aircraft that is powered by one or more JET engines. JET engines are a type of internal combustion engine that use air and fuel to create thrust. JET engines are used in many types of aircraft, including commercial airliners, military fighter jets, and private planes. The exhaust from a JET engine creates a contrail, which is a visible trail of condensed water vapor that forms behind the aircraft.