From Longman Dictionary of Contemporary Englishkeystonekey‧stone /ˈkiːstəʊn $ -stoʊn/ noun 1 TBC[countable] the large central stone in an arch that keeps the other stones in position2 IMPORTANT[singular] formal the most important part of an idea, belief, or process that influences how it developskeystone of The keystone of any personal injury case is medical evidence.
Examples from the Corpus
keystone of• A high level of production has become a keystone of economic security. 