First Lieutenant (1Lt) is the second lowest Commissioned Officer rank in the United States Air Force. It is just above Second Lieutenant and below Captain. First lieutenant is a commissioned officer military rank in many armed forces and, in some forces, an appointment. The rank of lieutenant has different meanings United Kingdom · Royal Navy · United States · U.S. Army, U.S. Marine. Example: First Lieutenant John Smith. The rank of first lieutenant corresponds to pay grade O Abbreviations: 1st Lieutenant, 1st Lt., 1st LT, and 1LT.