Propulsion is the action or process of pushing or pulling to drive an object forward. 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 (commonly called a powerplant), and wheels and axles, propellers, or a propulsive nozzle to generate the force. Components such as clutches or gearboxes may be needed to connect the motor to axles, wheels, or propellers.

Biological propulsion systems use an animal's muscles as the power source, and limbs such as wings, fins or legs as the propulsors.

A technological/biological system may use human, or trained animal, muscular work to power a mechanical device.

 

Note:   The above text is excerpted from the Wikipedia article Propulsion, which has been released under the GNU Free Documentation License.