The Collaborative International Dictionary of English v.0.48:

Police \Po*lice"\, n. [F., fr. L. politia the condition of a
   state, government, administration, Gr. ?, fr. ? to be a
   citizen, to govern or administer a state, fr. ? citizen, fr.
   ? city; akin to Skr. pur, puri. Cf. Policy polity,
   Polity.]
   1. A judicial and executive system, for the government of a
      city, town, or district, for the preservation of rights,
      order, cleanliness, health, etc., and for the enforcement
      of the laws and prevention of crime; the administration of
      the laws and regulations of a city, incorporated town, or
      borough.
      [1913 Webster]

   2. That which concerns the order of the community; the
      internal regulation of a state.
      [1913 Webster]

   3. The organized body of civil officers in a city, town, or
      district, whose particular duties are the preservation of
      good order, the prevention and detection of crime, and the
      enforcement of the laws.
      [1913 Webster]

   4. (Mil.) Military police, the body of soldiers detailed to
      preserve civil order and attend to sanitary arrangements
      in a camp or garrison.
      [1913 Webster]

   5. The cleaning of a camp or garrison, or the state ? a camp
      as to cleanliness.
      [1913 Webster]

   Police commissioner, a civil officer, usually one of a
      board, commissioned to regulate and control the
      appointment, duties, and discipline of the police.

   Police constable, or Police officer, a policeman.

   Police court, a minor court to try persons brought before
      it by the police.

   Police inspector, an officer of police ranking next below a
      superintendent.

   Police jury, a body of officers who collectively exercise
      jurisdiction in certain cases of police, as levying taxes,
      etc.; -- so called in Louisiana. --Bouvier.

   Police justice, or Police magistrate, a judge of a police
      court.

   Police offenses (Law), minor offenses against the order of
      the community, of which a police court may have final
      jurisdiction.

   Police station, the headquarters of the police, or of a
      section of them; the place where the police assemble for
      orders, and to which they take arrested persons.
      [1913 Webster]

WordNet (r) 3.0 (2006):

police constable
    n 1: a police officer of the lowest rank [syn: constable,
         police constable]