The Collaborative International Dictionary of English v.0.48:

Shire \Shire\, n. [AS. sc[imac]re, sc[imac]r, a division,
   province, county. Cf. Sheriff.]
   1. A portion of Great Britain originally under the
      supervision of an earl; a territorial division, usually
      identical with a county, but sometimes limited to a
      smaller district; as, Wiltshire, Yorkshire, Richmondshire,
      Hallamshire.
      [1913 Webster]

            An indefinite number of these hundreds make up a
            county or shire.                      --Blackstone.
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   2. A division of a State, embracing several contiguous
      townships; a county. [U. S.]
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   Note: Shire is commonly added to the specific designation of
         a county as a part of its name; as, Yorkshire instead
         of York shire, or the shire of York; Berkshire instead
         of Berks shire. Such expressions as the county of
         Yorkshire, which in a strict sense are tautological,
         are used in England. In the United States the composite
         word is sometimes the only name of a county; as,
         Berkshire county, as it is called in Massachusetts,
         instead of Berks county, as in Pensylvania.
         [1913 Webster]

               The Tyne, Tees, Humber, Wash, Yare, Stour, and
               Thames separate the counties of Northumberland,
               Durham, Yorkshire, Lincolnshire, etc. --Encyc.
                                                  Brit.
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   Knight of the shire. See under Knight.

   Shire clerk, an officer of a county court; also, an under
      sheriff. [Eng.]

   Shire mote (Old. Eng. Law), the county court; sheriff's
      turn, or court. [Obs.] --Cowell. --Blackstone.

   Shire reeve (Old Eng. Law), the reeve, or bailiff, of a
      shire; a sheriff. --Burrill.

   Shire town, the capital town of a county; a county town.

   Shire wick, a county; a shire. [Obs.] --Holland.
      [1913 Webster]

WordNet (r) 3.0 (2006):

shire town
    n 1: the town or city that is the seat of government for a shire
         [syn: county town, shire town]