prime mover

noun6

1. •
   The initial agent that is the cause of all things.
2. •
   A person or thing that is seminal and influential.

   • Seeger was a prime mover in the folk-music revival and was accepted by collegians and young people outside the South, regardless of his alleged Communist sympathies […] — 1986
3. •
   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
4. •
   Tractor unit. Or, more specifically, the front part of a semi-trailer type truck, i.e., the tractor to which the trailer part attaches.
5. •
   (US) A military or heavy construction vehicle.
6. •
   An agonist muscle.