prime mover

Short definition from Wiktionary

Noun

1. 1
   The initial agent that is the cause of all things.
2. 2
   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
3. 3
   The front part of a semi-trailer type truck, i.e., the tractor to which the trailer part attaches.
4. 4
   A military or heavy construction vehicle.
5. 5
   A person or thing that is seminal and influential.