keystone

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Architecture The central wedge-shaped stone of an arch that locks its parts together.
• noun The central supporting element of a whole.

from The Century Dictionary.

• noun 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.
• noun Hence A supporting principle; the chief element in a system; that upon which the remainder rests or depends.
• noun 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.
• noun 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

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun architecture The top stone of an arch.
• noun A native or resident of the American state of Pennsylvania.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the central building block at the top of an arch or vault
• noun a central cohesive source of support and stability