Hypertext Webster Gateway: "mainspring"
From Webster's Revised Unabridged Dictionary (1913) (web1913)
Mainspring \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.
From WordNet (r) 1.7 (wn)
mainspring
n : the most important spring in a mechanical device (especially
a clock or watch); as it uncoils it drives the mechanism
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