Posse comitatus

from 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]

from 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.