Source : Webster's Revised Unabridged Dictionary (1913)

Mainspring \Main"spring`\, n.
   The principal or most important spring in a piece of
   mechanism, especially the moving spring of a watch or clock
   or the spring in a gunlock which impels the hammer. Hence:
   The chief or most powerful motive; the efficient cause of
   action.

Source : WordNet®

mainspring
     n : the most important spring in a mechanical device (especially
         a clock or watch); as it uncoils it drives the mechanism