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English definition of “keystone”

keystone

noun [C] uk   /ˈkiː.stəʊn/ us    /-stoʊn/

keystone noun [C] (STONE)

the middle stone in the top of an arch that has a special shape and holds all the other stones in positionArches, columns and beams

keystone noun [C] (IMPORTANT PART)

the most important part of a plan, idea, etc. on which everything else dependsImportant and essential thingsInformation and messages
(Definition of keystone from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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