a rank in the military and maritime service (merchant marine), and the highest-ranking company officer. In most armies and in some air forces, a captain is the commander of the largest group of soldiers that an officer can be expected to know personally—a company in the infantry, a battery in the artillery, a flight in the air force.

On the sea a captain is usually the commander of a large warship—a cruiser, battleship, or aircraft carrier in the navy and any sizable ship in the mercantile marine service. In the British and U.S. navies the rank corresponds to the army rank of colonel, as does group captain in the Royal Air Force. An officer of lower rank is customarily given the courtesy title of captain when he is in command of a ship, so that he is addressed orally as captain, but he cannot claim the rank or be so addressed in writing. The position of commodore in the British Navy is not a separate rank but a special appointment of captain. In the U.S. Navy a commodore is ranked above a captain and below a rear admiral; the designation has usually been used only in wartime. Outside the navies, the master of any vessel is addressed as captain, and the term is usually applied as a courtesy to marine pilots.