Noun
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.
Source: Webster's dictionaryA chronometer differs from a spring-driven watch principally in that it contains a variable lever device to maintain even pressure on the mainspring, and a special balance designed to compensate for temperature variations. Source: Internet
“If the mainspring of popular government in peacetime is virtue, the mainspring of popular government in revolution is both virtue and terror: virtue without which terror is disastrous; terror without which virtue is powerless. Source: Internet
The electric clock's mainspring is wound either with an electric motor or with an electromagnet and armature. Source: Internet
In his Economic Harmonies, Bastiat states that, We cannot doubt that self-interest is the mainspring of human nature. Source: Internet
No, the mainspring of what we did was because it was fun." Source: Internet