The Collaborative International Dictionary of English v.0.48:

Commander \Com*mand"er\, n. [Cf. F. commandeur. Cf. Commodore,
   Commender.]
   1. A chief; one who has supreme authority; a leader; the
      chief officer of an army, or of any division of it.
      [1913 Webster]

            A leader and commander to the people. --Is. lv. 4.
      [1913 Webster]

   2. (Navy) An officer who ranks next below a captain, --
      ranking with a lieutenant colonel in the army.
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   3. The chief officer of a commandery.
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   4. A heavy beetle or wooden mallet, used in paving, in sail
      lofts, etc.
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   Commander in chief, the military title of the officer who
      has supreme command of the land or naval forces or the
      united forces of a nation or state; a generalissimo. The
      President is commander in chief of the army and navy of
      the United States.

   Syn: See Chief.
        [1913 Webster]

WordNet (r) 3.0 (2006):

commander
    n 1: an officer in command of a military unit [syn: commanding
         officer, commandant, commander]
    2: someone in an official position of authority who can command
       or control others
    3: a commissioned naval officer who ranks above a lieutenant
       commander and below a captain
    4: an officer in the airforce [syn: air force officer,
       commander]