Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

keystone
     n 1: a central cohesive source of support and stability; "faith
          is his anchor"; "the keystone of campaign reform was the
          ban on soft money"; "he is the linchpin of this firm"
          [syn: {anchor}, {mainstay}, {backbone}, {linchpin}, {lynchpin}]
     2: the central building block at the top of an arch or vault
        [syn: {key}, {headstone}]