lieutenant, company grade officer, the lowest rank of commissioned officer in most armies of the world. The lieutenant normally commands a small tactical unit such as a platoon.

In the British Army and in the United States Army, Air Force, and Marine Corps, a second lieutenant is the lowest ranking commissioned officer. Above him in those U.S. services comes a first lieutenant—lieutenant in the British Army—and then a captain. In the Russian Army there is still another rank, senior lieutenant. The term lieutenant has a somewhat different meaning in the U.S. and British navies, in which the lowest-ranking commissioned officer is an ensign (U.S.) or sublieutenant (British). The next higher rank is lieutenant junior grade (U.S. and British), followed by lieutenant and lieutenant commander. A U.S. Navy lieutenant is thus equal in rank to a U.S. Army, Air Force, or Marine Corps captain; a U.S. Navy ensign is equal in rank to a second lieutenant in the other services. In the Royal Air Force a flight lieutenant ranks below a squadron leader and above a flying officer.

The word also appears in combination with other military and civilian titles to denote a second-in-command or one of lower rank. A lieutenant colonel, for example, ranks below a colonel and above a major. A lieutenant general ranks below a general and above a major general. In the U.S. and British navies a lieutenant commander, as noted above, ranks between a lieutenant and a commander.