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 colonel
    n 1: a commissioned officer in the United States Army or Air
         Force or Marines holding a rank above major and below
         colonel [syn: lieutenant colonel, light colonel]