key‧stone /ˈkiːstəʊn $ -stoʊn/ noun

1. [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.

[TahlilGaran] Dictionary of Contemporary English ▲