The Collaborative International Dictionary of English v.0.48:

Primum mobile \Pri"mum mob"i*le\ [L., first cause of motion.]
   (Astron.)
   In the Ptolemaic system, the outermost of the revolving
   concentric spheres constituting the universe, the motion of
   which was supposed to carry with it all the inclosed spheres
   with their planets in a daily revolution from east to west.
   See Crystalline heavens, under Crystalline.
   [1913 Webster]

         The motions of the greatest persons in a government
         ought to be, as the motions of the planets, under
         primum mobile.                           --Bacon.
   [1913 Webster]

WordNet (r) 3.0 (2006):

primum mobile
    n 1: an agent that is the cause of all things but does not
         itself have a cause; "God is the first cause" [syn: first
         cause, prime mover, primum mobile]