Keystone

['kiːstəʊn] or ['ki'ston]

Definition

(noun.) the central building block at the top of an arch or vault.

Editor: Martin--From WordNet

Definition

(n.) The central or topmost stone of an arch. This in some styles is made different in size from the other voussoirs, or projects, or is decorated with carving. See Illust. of Arch.

Inputed by Hilary