D defs.my
Entry 1 sense Webster, 1913

mainspring

· main·spring · IPA
01 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…
  1. 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.(Fig.)