first lieutenant

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A commissioned rank in the US Army, Air Force, and Marines that is above second lieutenant and below captain.
• noun One who holds this rank.
• noun A lieutenant in the US Navy or Coast Guard, responsible for maintaining the ship's exterior and for management of the ship's deck.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The second lowest rank of a commissioned officer in the United States Army, Air Force, or Marine Corps, ranking above a second lieutenant and below a captain. The rank of first lieutenant is equivalent to the naval rank of lieutenant junior grade.
• noun An equivalent rank in other military forces.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a commissioned officer in the Army or Air Force or Marines ranking above a 2nd lieutenant and below a captain