1N-COUNT 拱顶石 A keystone is a stone at the top of an arch, which keeps the other stones in place by its weight and position. 拱顶石 [technical]
2N-COUNT [usually singular](政策、体系或进程的)基础,主旨 A keystone of a policy, system, or process is an important part of it, which is the basis for later developments. (政策、体系或进程的)基础,主旨
Keeping inflation low is the keystone of their economic policy. [+ of/in]