The Collaborative International Dictionary of English v.0.48:

Motive \Mo"tive\, a.
   Causing motion; having power to move, or tending to move; as,
   a motive argument; motive power. "Motive faculty." --Bp.
   Wilkins.
   [1913 Webster]

   Motive power (Mach.), a natural agent, as water, steam,
      wind, electricity, etc., used to impart motion to
      machinery; a motor; a mover.
      [1913 Webster]

WordNet (r) 3.0 (2006):

motive power
    n 1: the power or ability to move [syn: locomotion, motive
         power, motivity]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "motive power":
   activator, animator, appliance, arouser, bunt, butt, clout,
   compulsion, convenience, drive, driving force, energizer, engine,
   enginery, facility, fixture, human dynamo, impellent,
   impelling force, impetus, impulse, impulsion, incentive,
   incitement, irresistible force, life, locomotion, machine,
   machinery, mechanical aid, mechanical device, mechanism, mobility,
   moment, momentum, motility, motivating force, motivity, motor,
   movableness, power, power plant, power source, propelling,
   propelment, propulsion, pulsion, push, pushing, restorative, shove,
   shoving, shunt, spark plug, stimulant, stimulator, stimulus,
   thrust, tonic, utility