power unit

noun power units

1. A truck (large road vehicle) capable of moving under its own power. The combination unit was traversing a straight section of roadway, on an approximate 4 percent upgrade, when the tractor began to jackknife. As the power unit rotated clockwise, it left the roadway to the right and traveled several feet before the driver corrected the jackknife. The unit was redirec
2. Any other unit capable of providing power. For many years the horse provided a cheap and effective power unit for town cartage, but in recent years it has been replaced largely by mechanical power.