mainspring
noun
main·spring ˈmān-ˌspriŋ 
1
: the chief spring in a mechanism especially of a watch or clock
2
: the chief or most powerful motive, agent, or cause
Example Sentences
Agriculture is the mainspring of their economy.
Recent Examples on the Web An utterly absurd car chase and a fight ensue—so does a warm personal relationship between Emily and Youcef, which provides the mainspring of the movie’s drama. 
Richard Brody, The New Yorker, 11 Aug. 2022 Here was an opportunity to acquire and restore the old mainspring. Gregory Thomas, San Francisco Chronicle, 3 Aug. 2022 Inside, there are two mainplates, each of which holds a movement consisting of a mainspring, a cylinder, a comb and regulator. Roberta Naas, Forbes, 4 May 2022 The energy created by these thermal variations is connected to a mechanism that uses it to wind the mainspring. Carol Besler, Robb Report, 13 Apr. 2022 His musical collaboration with Parks is the personal, passionate mainspring of that transformation. Richard Brody, The New Yorker, 26 Apr. 2022 Cartier has created a platinum automatic skeleton watch in which the rotor houses the entire movement—mainspring, escapement, and all. Charles Curkin, ELLE Decor, 12 Apr. 2022 Her film’s ironies start with the title, because many of the movie’s viewers, like many of its interview subjects from the world of music, would rather not listen to Kenny G’s music at all—and their aversion is the mainspring of the film. Richard Brody, The New Yorker, 4 Dec. 2021 Between picturesque villages with draconian speed limits, the iX unwinds like a fine mainspring. Dan Neil, WSJ, 1 Oct. 2021 See More
Richard Brody, The New Yorker, 11 Aug. 2022 Here was an opportunity to acquire and restore the old mainspring. Gregory Thomas, San Francisco Chronicle, 3 Aug. 2022 Inside, there are two mainplates, each of which holds a movement consisting of a mainspring, a cylinder, a comb and regulator. Roberta Naas, Forbes, 4 May 2022 The energy created by these thermal variations is connected to a mechanism that uses it to wind the mainspring. Carol Besler, Robb Report, 13 Apr. 2022 His musical collaboration with Parks is the personal, passionate mainspring of that transformation. Richard Brody, The New Yorker, 26 Apr. 2022 Cartier has created a platinum automatic skeleton watch in which the rotor houses the entire movement—mainspring, escapement, and all. Charles Curkin, ELLE Decor, 12 Apr. 2022 Her film’s ironies start with the title, because many of the movie’s viewers, like many of its interview subjects from the world of music, would rather not listen to Kenny G’s music at all—and their aversion is the mainspring of the film. Richard Brody, The New Yorker, 4 Dec. 2021 Between picturesque villages with draconian speed limits, the iX unwinds like a fine mainspring. Dan Neil, WSJ, 1 Oct. 2021 See More Word History
First Known Use
1591, in the meaning defined at sense 1
Kids Definition
mainspring
noun
main·spring ˈmān-ˌspriŋ
: the principal spring in a mechanical device (as a watch or clock)