What does KEY stand for?

Keystone

The centre stone of an arch, if it is larger than the ordinary voussoirs. 1.Stone in the form of wedge forming the central element of a lintel, vault, or arch. Pendant keystones are found at the intersection of ribbed vaults. Originally from the Latin clavis for "key". 2.The central wedge-shaped member of a masonry arch; also used as a decorative element on arches in wood structures.