The Collaborative International Dictionary of English v.0.48:

Posse comitatus \Pos"se com`i*ta"tus\ [L. posse to be able, to
   have power + LL. comitatus a county, from comes, comitis, a
   count. See County, and Power.]
   [1913 Webster]
   1. (Law) The power of the county, or the citizens who may be
      summoned by the sheriff to assist the authorities in
      suppressing a riot, or executing any legal precept which
      is forcibly opposed. --Blackstone.
      [1913 Webster]

   2. A collection of people; a throng; a rabble. [Colloq.]
      [1913 Webster]

   Note: The word comitatus is often omitted, and posse alone
         used. "A whole posse of enthusiasts." --Carlyle.
         [1913 Webster]

               As if the passion that rules were the sheriff of
               the place, and came off with all the posse.
                                                  --Locke.
         [1913 Webster]

WordNet (r) 3.0 (2006):

posse comitatus
    n 1: a temporary police force [syn: posse, posse comitatus]