The Collaborative International Dictionary of English v.0.48:

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:
   (Fig.) The chief or most powerful motive; the efficient cause
   of action; as, the mainspring of action.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mainspring
    n 1: the most important spring in a mechanical device
         (especially a clock or watch); as it uncoils it drives the
         mechanism