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Definition of keystone noun from the Oxford Advanced Learner's Dictionary

keystone

noun
 
/ˈkiːstəʊn/
 
/ˈkiːstəʊn/
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  1. (architecture) the central stone at the top of an arch that keeps all the other stones in positionTopics Buildingsc2
  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.
    Topics Opinion and argumentc2
See keystone in the Oxford Advanced American Dictionary
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halfway
adverb
 
 
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