Definition of mainspring noun from the Oxford Advanced American Dictionary

     

    mainspring

     noun
    noun
    NAmE//ˈmeɪnsprɪŋ//
     
     
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  1. 1 [usually singular] mainspring (of something) (formal) the most important part of something; the most important influence on something Small companies are the mainspring of the American economy. Her Christian faith was the mainspring of her life.
  2. 2the most important spring in a watch, clock, etc.
See the Oxford Advanced Learner's Dictionary entry: mainspring