prime mover

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One regarded as the initial source of energy directed toward a goal.
• noun The initial force, such as electricity, wind, or gravity, that engages or moves a machine.
• noun A machine or mechanism that converts natural energy into work.
• noun Any of various heavy-duty trucks or tractors.
• noun Philosophy In Aristotelian philosophy, the self-moved being that causes all motion.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun philosophy The initial agent that is the cause of all things.
• noun A machine, such as a water wheel or steam engine, that receives and modifies energy as supplied by some natural source or fuel and transforms it into mechanical work
• noun automotive The front part of a semi-trailer type truck, i.e., the tractor to which the trailer part attaches.
• noun US A military or heavy construction vehicle.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an agent that is the cause of all things but does not itself have a cause