Mainspring

Full definition of mainspring

Noun

1. The principal spring of a clockwork mechanism, that drives it by uncoiling.
• 1922, Margery Williams, The Velveteen RabbitHe was wise, for he had seen a long succession of mechanical toys arrive to boast and swagger, and by-and-by break their mainsprings and pass away.
2. (figuratively) The most important reason for something.My daughter is the mainspring of my life.