keystone

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Architecture The central wedge-shaped stone of an arch that locks its parts together. Also called headstone.
2. n. The central supporting element of a whole.

Century Dictionary and Cyclopedia

1. n. The stone of an arch (typically the uppermost stone), which, being the last put in, is regarded as keying or locking the whole structure together; the stone at the apex of an arch. in Roman and Renaissance arches the keystone is very commonly sculptured as a decorative feature. In groined medieval vaults the keystone at the intersection of the ribs at the summit of the vault is usually ornamented with a sculptured boss or pendant. In a true or extradosed arch no one of the voussoirs is more important to the stability of the structure than any other. See arch, n., 2.
2. n. Hence A supporting principle; the chief element in a system; that upon which the remainder rests or depends.
3. n. In chromolithography, the first stone on which the picture is drawn or photographed, to serve as an outline guide in preparing the other stones for the colors, a copy of the keystone being made on each stone for printing a single color. See lithography.
4. n. In a Scotch lead-smelting furnace, a block of cast-iron used to close up the space at each end of the forestone, and to fill up the space between the forestone and the back part of the furnace

Wiktionary

1. n. The top stone of an arch.
2. n. A native or resident of the American state of Pennsylvania.

GNU Webster's 1913

1. 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.

WordNet 3.0

1. n. the central building block at the top of an arch or vault
2. n. a central cohesive source of support and stability