Propulsion is a means of creating force leading to movement. The term is derived from two Latin words: pro, meaning before or forward; and pellere, meaning to drive. A propulsion system consists of a source of mechanical power, and a propulsor (means of converting this power into propulsive force). A technological system uses an engine or motor as the power source, and wheels and axles, propellers, or a propulsive nozzle to generate the force.