The Collaborative International Dictionary of English v.0.48:

Commissioner \Com*mis"sion*er\, n.
   1. A person who has a commission or warrant to perform some
      office, or execute some business, for the government,
      corporation, or person employing him; as, a commissioner
      to take affidavits or to adjust claims.
      [1913 Webster]

            To another address which requested that a commission
            might be sent to examine into the state of things in
            Ireland, William returned a gracious answer, and
            desired the Commons to name the commissioners.
                                                  --Macaulay.
      [1913 Webster]

   2. An officer having charge of some department or bureau of
      the public service.
      [1913 Webster]

            Herbert was first commissioner of the Admiralty.
                                                  --Macaulay.
      [1913 Webster]

            The commissioner of patents, the commissioner of the
            land office, the commissioner of Indian affairs, are
            subordinates of the secretary of the interior.
                                                  --Bartlett.
      [1913 Webster]

   Commissioner of deeds, an officer having authority to take
      affidavits, depositions, acknowledgment of deeds, etc.,
      for use in the State by which he is appointed. [U. S.]

   County commissioners, certain administrative officers in
      some of the States, invested by local laws with various
      powers in reference to the roads, courthouses, financial
      matters, etc., of the county. [U. S.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

commissioner
    n 1: a government administrator
    2: a member of a commission

Moby Thesaurus II by Grady Ward, 1.0:

89 Moby Thesaurus words for "commissioner":
   G-man, MP, alderman, archon, bailie, bailiff, beadle, beagle,
   bound bailiff, burghermaster, burgomaster, cabinet member,
   cabinet minister, captain, catchpole, chancellor, chief of police,
   city councilman, city father, city manager, commissar, commissary,
   commissionaire, constable, councillor, councilman, councilwoman,
   county commissioner, county supervisor, delegate, deputy,
   deputy sheriff, detective, elder, emissary, envoy, fed, federal,
   flic, gendarme, government man, headman, herald, induna, inspector,
   legate, legislator, lictor, lieutenant, lord mayor, mace-bearer,
   magistrate, maire, marshal, mayor, messenger, minister,
   minister of state, mounted policeman, narc, officer, patrolman,
   peace officer, police captain, police commissioner,
   police constable, police inspector, police matron, police officer,
   police sergeant, policeman, policewoman, portreeve, reeve,
   roundsman, secretary, secretary of state, selectman, sergeant,
   sergeant at arms, sheriff, superintendent, supervisor, syndic,
   tipstaff, tipstaves, trooper, undersecretary, warden



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

COMMISSIONER, officer. One who has a lawful commission to execute a public
office. In a more restricted sense it is one who is authorized to execute. a
particular duty, as, commissioner of the revenue, canal commissioner. The
term when used in this latter sense is not applied, for example, to a judge.
There are commissioners, too, who have no regular commissions and derive
their author from the elections held by the people. County commissioners, in
Pennsylvania, are officers of the latter kind.