Keystone State

from WordNet (r) 3.0 (2006)

Keystone State
    n 1: a Mid-Atlantic state; one of the original 13 colonies [syn:
         {Pennsylvania}, {Keystone State}, {PA}]

from The Collaborative International Dictionary of English v.0.48

Keystone State \Key"stone` State\
   Pennsylvania; -- a nickname alluding to its having been the
   central one of the 13 original United States, at the time of
   formation of the Constitution.
   [Webster 1913 Suppl.]