The Collaborative International Dictionary of English v.0.48:

Sergeant \Ser"geant\, n. [F. sergent, fr. L. serviens, -entis,
   p. pr. of servire to serve. See Serve, and cf. Servant.]
   [Written also serjeant. Both spellings are authorized. In
   England serjeant is usually preferred, except for military
   officers. In the United States sergeant is common for civil
   officers also.]
   1. Formerly, in England, an officer nearly answering to the
      more modern bailiff of the hundred; also, an officer whose
      duty was to attend on the king, and on the lord high
      steward in court, to arrest traitors and other offenders.
      He is now called sergeant-at-arms, and two of these
      officers, by allowance of the sovereign, attend on the
      houses of Parliament (one for each house) to execute their
      commands, and another attends the Court Chancery.
      [1913 Webster]

            The sergeant of the town of Rome them sought.
                                                  --Chaucer.
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            The magistrates sent the serjeant, saying, Let those
            men go.                               --Acts xvi.
                                                  35.
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            This fell sergeant, Death,
            Is strict in his arrest.              --Shak.
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   2. (Mil.) In a company, battery, or troop, a noncommissioned
      officer next in rank above a corporal, whose duty is to
      instruct recruits in discipline, to form the ranks, etc.
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   Note: In the United States service, besides the sergeants
         belonging to the companies there are, in each regiment,
         a sergeant major, who is the chief noncommissioned
         officer, and has important duties as the assistant to
         the adjutant; a quartermaster sergeant, who assists the
         quartermaster; a color sergeant, who carries the
         colors; and a commissary sergeant, who assists in the
         care and distribution of the stores. Ordnance sergeants
         have charge of the ammunition at military posts.
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   3. (Law) A lawyer of the highest rank, answering to the
      doctor of the civil law; -- called also serjeant at law.
      [Eng.] --Blackstone.
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   4. A title sometimes given to the servants of the sovereign;
      as, sergeant surgeon, that is, a servant, or attendant,
      surgeon. [Eng.]
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   5. (Zool.) The cobia.
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   Drill sergeant. (Mil.) See under Drill.

   Sergeant-at-arms, an officer of a legislative body, or of a
      deliberative or judicial assembly, who executes commands
      in preserving order and arresting offenders. See
      Sergeant, 1.

   Sergeant major.
      (a) (Mil.) See the Note under def. 2, above.
      (b) (Zool.) The cow pilot.
          [1913 Webster]

WordNet (r) 3.0 (2006):

sergeant
    n 1: any of several noncommissioned officer ranks in the Army or
         Air Force or Marines ranking above a corporal
    2: a lawman with the rank of sergeant [syn: police sergeant,
       sergeant]
    3: an English barrister of the highest rank [syn: serjeant-at-
       law, serjeant, sergeant-at-law, sergeant]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "sergeant":
   G-man, MP, NCO, acting corporal, acting sergeant, bailiff, beadle,
   beagle, bound bailiff, captain, catchpole, centurion,
   chief of police, chief warrant officer, color sergeant,
   commissioner, constable, corporal, deputy, deputy sheriff,
   detective, fed, federal, first sergeant, flic, gendarme,
   government man, havildar, inspector, lance corporal,
   lance sergeant, lictor, lieutenant, mace-bearer, marshal,
   master sergeant, mess sergeant, mounted policeman, naik, narc,
   noncom, noncommissioned officer, officer, patrolman, peace officer,
   platoon sergeant, police captain, police commissioner,
   police constable, police inspector, police matron, police officer,
   police sergeant, policeman, policewoman, portreeve, reeve,
   roundsman, sarge, sergeant at arms, sergeant first class,
   sergeant major, sheriff, staff sergeant, superintendent,
   technical sergeant, tipstaff, tipstaves, top sergeant, topkick,
   trooper, warrant officer



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

SERGEANT or SERJEANT, Eng. law. An officer in the courts of the highest
grade among the practitioners of the law.



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

SERGEANT or SERJEANT, in the army. An inferior officer of a company of foot,
or troop of dragoons appointed to see discipline observed, to teach the
soldiers the exercise of their arms, and to order, straighten and form
ranks, files, &c.