Longman Dictionary of Contemporary English homepage

keystone

 noun
     
Related topics: Construction
key‧stone
1TBC [countable] the large central stone in an arch that keeps the other stones in position
2 [singular] formal the most important part of an idea, belief, or process that influences how it develops
keystone of
The keystone of any personal injury case is medical evidence.
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.

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