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]