mainspring
noun
- spiral torsion spring of metal ribbon used as a power source in mechanical watches, some clocks, and other clockwork mechanisms
Wiktionary
noun
Etymology: From main + spring. For figurative sense, compare spring (“source, origin”).
- 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.”
“But in the England of Shakespeare’s history plays, the mainspring of the system has broken down.”