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]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

POSSE COMITATUS. These Latin words signify the power of the county.
     2. The sheriff has authority by the common law, while acting under the
authority of the writ of the United States, commonwealth or people, as the
case may be, and for the purpose of preserving the public peace, to call to
his aid the posse comitatus.
     3. But with respect to writs which issue, in the first instance, to
arrest in civil suits, the sheriff is not bound to take the posse comitatus
to assist him in the execution of them: though he may, if he pleases, on
forcible resistance to the execution of the process. 2 Inst. 193; 3 Inst.
161.
     4. Having the authority to call in the assistance of all, it seems to
follow, that he may equally require that of any individual; but to this
general rule there are some exceptions; persons of infirm health, or who
want understanding, minors under the age of fifteen years, women, and
perhaps some others, it seems, cannot be required to assist the sheriff, and
are therefore not considered as a part of the power of the county. Vin. Ab.
Sheriff, B.
     5. A refusal on the part of an individual lawfully called upon to
assist the officer in putting down a riot is indictable. 1 Carr. & Marsh.
314. In this case will be found the form of an indictment for this offence.
     6. Although the sheriff is acting without authority, yet it would seem
that any person who obeys his command, unless aware of that fact, will be
protected.
     7. Whether an individual not enjoined by the sheriff to lend his aid,
would be protected in his interference, seems questionable. In a case where
the defendant assisted sheriff's officers in executing a writ of replevin
without their solicitation, the court held him justified in so doing. 2 Mod.
244. Vide Bac. Ab. Sheriff, N; Hamm. N. P. 63; 5 Whart. R. 437, 440.