mainspring

Etymology

From main + spring. For figurative sense, compare spring (“source, origin”).

mainspring means the principal spring of a clockwork mechanism, which drives it by uncoiling. Lexicurio rates it Sui generis — a strength score of 86 out of 100.

Why this word is great

MAINSPRING — [Noun] The principal spring in a mechanism, such as a watch, that drives it by uncoiling, or, figuratively, the most important motivating force or reason for something. From main ("principal, chief") + spring ("a resilient coil or, figuratively, source, origin"). Unlike "motive," which is a conscious impulse for a specific act, or "catalyst," which is an external spark for change, the mainspring is the inherent, coiled potential within—the tireless, internal driver of the whole. It is the tempered steel spiral at the heart of a grandfather clock, the quiet obsession structuring a decade of research, and the deep-seated love that powers a family through decades. The true mainspring is the concealed, tensioned truth from which all visible motion unspools, and its breaking is the definition of stillness.

noun

The principal spring of a clockwork mechanism, which drives it by uncoiling.“He was wise, for he had seen a long succession of mechanical toys arrive to boast and swagger, and by-and-by break their mainsprings and pass away.”
The most important reason for, or element of, something.“My daughter is the mainspring of my life.”