Definitions for: Mainspring

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

Webster (1913) Definition: 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:
The chief or most powerful motive; the efficient cause of
action.

See Also: clockwork, spring