Lieutenant, a commissioned officer. In the United States Army, Air Force, and Marine Corps, lieutenants are the lowest-ranking commissioned officers. There are two grades, second lieutenant (the lower) and first lieutenant. In the Navy and Coast Guard, lieutenants rank above ensigns and below lieutenant commanders. Lieutenant junior grade is equivalent to an Army first lieutenant, and lieutenant is equivalent to an Army captain Originally the term lieutenant referred to any official, civil or military, who took the place of a superior.