Available Definitions:
1) n. - An officer who supplies the place of a superior in his absence; a representative of, or substitute for, another in the performance of any duty.
2) n. - A commissioned officer in the army, next below a captain.
3) n. - A commissioned officer in the British navy, in rank next below a commander.
4) n. - A commissioned officer in the United States navy, in rank next below a lieutenant commander.