colonel, the highest field-grade officer, ranking just below the general officer grades in most armies or below brigadier in the British services. A colonel was traditionally the commanding officer of a regiment or brigade. In air forces that use the same titles of rank as the army, such as the U.S. Air Force, a colonel’s command is usually a group; the comparable grade in the Royal Air Force is group captain. When not exercising command of a regiment, group, or equivalent formation, a colonel is generally placed in a senior staff or administrative post.