mainspring

n.

1.

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.

Noun

1.

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

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