Source : Webster's Revised Unabridged Dictionary (1913)

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}.]
   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.

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

   Note: The word comitatus is often omitted, and posse alone
         used. ``A whole posse of enthusiasts.'' --Carlyle.

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

Source : WordNet®

posse comitatus
     n : a temporary police force [syn: {posse}]