Commander

1. A chief; one who has supreme authority; a leader; the chief officer of an army, or of any division of it.
2. (Navy) An officer who ranks next below a captain, -- ranking with a lieutenant colonel in the army.
3. The chief officer of a commandery.
4. A heavy beetle or wooden mallet, used in paving, in sail lofts, etc.
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.