- (
architecture 建筑 the central stone at the top of an arch that keeps all the other stones in position 拱顶石 
- [usually singular]
the most important part of a plan or argument that the other parts depend on （计划、论据的）主旨，基础 Changes to the welfare system are the keystone of the government’s reforms. 改变福利制度是政府改革的基石。