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English definition of “keystone”

keystone

noun [C] (STONE)    /ˈkiː.stəʊn/ US  /-stoʊn/
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
(Definition of keystone noun (STONE) from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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