capstone
/ˈkæpstəʊn/noun
1- a stone fixed on top of something, typically a wall（尤指墙的）拱顶石, 压顶石, 顶石。
1.1
- Archaeology a large flat stone forming a roof over the chamber of a megalithic tomb【考古】墓顶石（巨石墓室的平顶）。