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

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "mainspring":
   ambition, aspiration, basis, calling, cause, consideration, font,
   fount, fountain, fountainhead, goal, ground, guiding light,
   guiding star, headspring, headstream, headwater, ideal,
   inspiration, intention, lodestar, matter, mine, motive, principle,
   quarry, reason, riverhead, sake, score, source, spring, springhead,
   ulterior motive, vocation, well, wellhead, wellspring