A motor or motor is a machine designed to convert a form of mechanical energy into energy.  Heat engines convert heat to work through various thermodynamic processes.

The internal combustion engine, perhaps also heat from a heat engine, is the most common example of combustion of a fuel in the combustion chamber, which leads to the rapid pressure of gaseous combustion products, causing them to expand and drive a piston, rotating a crank. Electric motors convert into electrical energy mechanical motion, pneumatic motors use compressed air and winding toys as clockwork motors use elastic energy. In biological systems, molecular motors such as myosins in muscles, use chemical energy to generate forces and ultimately movement.

The word engine is derived from Old French, from the vast Latin word INGENIUM - the root subtly. Pre-war industrial weapons, such as catapults, trebuchets, and rams, were called siege engines, and information on how to create them was mostly treated as military secrets.

The word gin is an abbreviation for motor, as in cotton gin. Most mechanical devices invented during the industrial revolution were described as motors - the steam engine is a notable example. However, the original steam engines like Thomas Savery's were pumps, not mechanical engines. In this way, a fire truck in its original form consisted of just a water pump and the engine was carried into the fire by horses.