1. A person who commands, especially a commanding officer.
2. Abbr. CDR, Cdr., Cmd., Cmdr, com., Com., Comdr. a. A commissioned rank in the U.S. Navy or Coast Guard that is above lieutenant commander and below captain. b. One who holds this rank. c. The chief commissioned officer of a military unit regardless of his or her rank.
3. An officer in some knightly or fraternal orders.