Definitions
1.nounAn army officer in most countries, ranking below a colonel and above a major. Often the commander of a battalion or the executive officer of a regiment.
2.nounAn officer in the United States Marine Corps or Royal Marines, of equivalent rank to their counterparts in the army.
3.nounAn officer in the United States Air Force, Air Force Reserve, or Air National Guard, ranking below colonel and above major. Often the commander of a squadron or the executive officer of a wing.