keystone
/ˈkiːstəʊn/noun
1- a central stone at the summit of an arch, locking the whole together拱顶石。
1.1
- usu. in sing. the central principle or part of a policy, system, etc., on which all else depends基础; 主旨; 基本原则：
cooperation remains the keystone of the government's security policy.
合作仍然是政府安全政策的基本原则。