WordNet (r) 3.0 (2006):

driving force
    n 1: the act of applying force to propel something; "after
         reaching the desired velocity the drive is cut off" [syn:
         drive, thrust, driving force]

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "driving force":
   bunt, butt, clout, compulsion, drive, impellent, impelling force,
   impetus, impulse, impulsion, incentive, incitement,
   irresistible force, moment, momentum, motive power, power,
   propelling, propelment, propulsion, pulsion, push, pushing, shove,
   shoving, shunt, thrust