mainspring

noun

1. the principal spring in a mechanism, as in a watch.
2. the chief motive power; the impelling cause.

noun

1. the principal power spring of a mechanism, esp in a watch or clock
2. the chief cause or motive of something

n.1590s, of watches, clocks, etc., from main (adj.) + spring (n.3). Figurative use from 1690s.