- U.S. Mil. an officer ranking above a second lieutenant and below a captain
- U.S. Navy the officer in charge of maintenance of a ship or shore station
- A commissioned rank in the US Army, Air Force, and Marines that is above second lieutenant and below captain.
- One who holds this rank.
- A lieutenant in the US Navy or Coast Guard, responsible for maintaining the ship's exterior and for management of the ship's deck.
(plural first lieutenants)
When used as a title, it is always capitalized.
- Example: First Lieutenant John Smith. The rank of first lieutenant corresponds to pay grade O-2.
Abbreviations: 1st Lieutenant, 1st Lt., 1st LT, and 1LT.