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.
      [1913 Webster]

   3. The chief officer of a commandery.
      [1913 Webster]

   4. A heavy beetle or wooden mallet, used in paving, in sail
      lofts, etc.
      [1913 Webster]

   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]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "commander":
   A per se, ADC, CO, OD, ace, admiral, aide, aide-de-camp, boatswain,
   boss, brigadier, brigadier general, captain, castellan, champion,
   chatelain, chatelaine, chicken colonel, chief, chief engineer,
   chief mate, chief of staff, chief petty officer, colonel,
   commandant, commander in chief, commanding officer,
   commissioned officer, commodore, company officer, dean,
   deck officer, director, ensign, exec, executive, executive officer,
   field marshal, field officer, first lieutenant, five-star general,
   fleet admiral, four-star general, fugleman, general,
   general officer, generalissimo, genius, governor, head, higher-up,
   intendant, jemadar, junior officer, laureate, leader, lieutenant,
   lieutenant colonel, lieutenant commander, lieutenant general,
   lieutenant junior grade, major, major general, manager, marechal,
   marshal, master, mate, naval officer, navarch, navigating officer,
   navigator, nonpareil, officer, one-star general, orderly officer,
   paragon, patron, petty officer, pipes, principal, prodigy,
   quartermaster, rear admiral, risaldar, ruler, sailing master,
   second mate, senior, senior officer, shavetail, shipmaster, sirdar,
   skipper, staff officer, star, subahdar, subaltern, sublieutenant,
   superior, superman, superstar, the Old Man, the brass,
   the greatest, the most, three-star general, top brass, top dog,
   two-star general, vice admiral, virtuoso, warrant officer,
   watch officer