The Collaborative International Dictionary of English v.0.48:

Lieutenant \Lieu*ten"ant\ (l[-u]*t[e^]n"ant), n. [F., fr. lieu
   place + tenant holding, p. pr. of tenir to hold, L. tenere.
   See Lieu, and Tenant, and cf. Locum tenens.]
   1. An officer who supplies the place of a superior in his
      absence; a representative of, or substitute for, another
      in the performance of any duty.
      [1913 Webster]

            The lawful magistrate, who is the vicegerent or
            lieutenant of God.                    --Abp.
                                                  Bramhall.
      [1913 Webster]

   2.
      (a) A commissioned officer in the army, next below a
          captain.
      (b) A commissioned officer in the British navy, in rank
          next below a commander.
      (c) A commissioned officer in the United States navy, in
          rank next below a lieutenant commander.
          [1913 Webster]

   Note: Lieutenant is often used, either adjectively or in
         hyphened compounds, to denote an officer, in rank next
         below another, especially when the duties of the higher
         officer may devolve upon the lower one; as, lieutenant
         general, or lieutenant-general; lieutenant colonel, or
         lieutenant-colonel; lieutenant governor, etc.
         [1913 Webster]

   Deputy lieutenant, the title of any one of the deputies or
      assistants of the lord lieutenant of a county. [Eng.]

   Lieutenant colonel, an army officer next in rank above
      major, and below colonel.

   Lieutenant commander, an officer in the United States navy,
      in rank next below a commander and next above a
      lieutenant.

   Lieutenant general. See in Vocabulary.

   Lieutenant governor.
      (a) An officer of a State, being next in rank to the
          governor, and in case of the death or resignation of
          the latter, himself acting as governor. [U. S.]
      (b) A deputy governor acting as the chief civil officer of
          one of several colonies under a governor general.
          [Eng.]
          [1913 Webster]

WordNet (r) 3.0 (2006):

lieutenant
    n 1: a commissioned military officer
    2: an officer in a police force [syn: lieutenant, police
       lieutenant]
    3: an assistant with power to act when his superior is absent
       [syn: deputy, lieutenant]
    4: an officer holding a commissioned rank in the United States
       Navy or the United States Coast Guard; below lieutenant
       commander and above lieutenant junior grade

Moby Thesaurus II by Grady Ward, 1.0:

162 Moby Thesaurus words for "lieutenant":
   ADC, CO, G-man, MP, OD, acolyte, adjutant, admiral, advocate,
   agent, aid, aide, aide-de-camp, aider, alter ego, alternate,
   amicus curiae, assistant, attendant, attorney, auxiliary, backup,
   backup man, bailiff, beadle, beagle, best man, bound bailiff,
   brigadier, brigadier general, captain, catchpole, champion,
   chicken colonel, chief of police, chief of staff,
   chief petty officer, coadjutant, coadjutor, coadjutress,
   coadjutrix, colonel, commandant, commander, commander in chief,
   commanding officer, commissioned officer, commissioner, commodore,
   company officer, constable, deputy, deputy sheriff, detective,
   dummy, ensign, exec, executive officer, exponent, fed, federal,
   field marshal, field officer, figurehead, first lieutenant,
   five-star general, fleet admiral, flic, four-star general,
   gendarme, general, general officer, generalissimo, government man,
   help, helper, helpmate, helpmeet, inspector, jemadar,
   junior officer, lictor, lieutenant colonel, lieutenant commander,
   lieutenant general, lieutenant junior grade, locum, locum tenens,
   mace-bearer, major, major general, marechal, marshal,
   mounted policeman, narc, naval officer, navarch, officer,
   one-star general, orderly officer, paranymph, paraprofessional,
   patrolman, peace officer, petty officer, pinch hitter, pleader,
   police captain, police commissioner, police constable,
   police inspector, police matron, police officer, police sergeant,
   policeman, policewoman, portreeve, procurator, proxy, rear admiral,
   reeve, representative, risaldar, roundsman, second,
   second in command, secondary, senior officer, sergeant,
   sergeant at arms, servant, shavetail, sheriff, sideman, sirdar,
   skipper, staff officer, stand-in, subahdar, subaltern,
   sublieutenant, substitute, superintendent, supporting actor,
   supporting instrumentalist, surrogate, the Old Man, the brass,
   three-star general, tipstaff, tipstaves, top brass, trooper,
   two-star general, understudy, utility man, vicar, vicar general,
   vice, vice admiral, vicegerent, warrant officer



Easton's 1897 Bible Dictionary:

Lieutenant
   (only in A.V. Esther 3:12; 8:9; 9:3; Ezra 8:36), a governor or
   viceroy of a Persian province having both military and civil
   power. Correctly rendered in the Revised Version "satrap."


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

LIEUTENANT. This word has now a narrower meaning than it formerly had; its
true meaning is a deputy, a substitute, from the French lieu, (place or
post) and tenant (holder). Among civil officers we have lieutenant
governors, who in certain cases perform the duties of governors; (vide, the
names of the several states,) lieutenants of police, &c. Among military men,
lieutenant general was formerly the title of a commanding general, but now
it signifies the degree above major general. Lieutenant colonel, is the
officer between the colonel and the major. Lieutenant simply signifies the
officer next below a captain. In the navy, a lieutenant is the second
officer next in command to the captain of a ship.