| Mainspring (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. Thesaurus Entries 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 |