1. Someone in an official position of authority who can control others: Sometimes the principal of a school acts as a commander when there are disciplinary problems among the students and he or she can demand that they do what they are told.
2. A person who exercises authority; a chief officer; a leader: The commander of the local police force was well-liked for being fair and protective of the citizens of his community. 
3. An officer ranking below a captain and above a lieutenant commander in a military unit: Joan's brother became a commander after serving successfully during his ten years in the navy.