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.

Keystone State \Key"stone` State\
   Pennsylvania; -- a nickname alluding to its having been the
   central one of the 13 original United States.

Source : WordNet®

Keystone State
     n : a Mid-Atlantic state; one of the original 13 colonies [syn:
         {Pennsylvania}, {PA}]