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Definition of keystone noun from the Oxford Advanced American Dictionary

     

    keystone

     noun
    noun
    NAmE//ˈkistoʊn//
     
     
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  1. 1(architecture) the central stone at the top of an arch that keeps all the other stones in position
  2. 2[usually singular] the most important part of a plan or argument that the other parts depend on Changes to the welfare system are the keystone of the government's reforms.
See keystone in the Oxford Advanced Learner's Dictionary
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