second lieutenant

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

• noun The lowest commissioned rank in the US Army, Air Force, and Marine Corps.
• noun One who holds this rank.

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

• noun The lowest rank of a commissioned army officer, below a lieutenant.
• noun The lowest rank of a commissioned officer in the United States Army, United States Air Force, or United States Marine Corps, below a first lieutenant. The rank of second lieutenant is equivalent to the naval rank of ensign.

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

• noun a commissioned officer in the Army or Air Force or Marine Corps holding the lowest rank