Keystone \Key"stone`\, n. (Arch.) 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.

Keystone State, the State of Pennsylvania; -- so called from its having been the central State of the Union at the formation of the Constitution.

Source: Webster's Revised Unabridged Dictionary (1913)