lieutenant junior grade

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

• noun A commissioned rank in the US Navy or Coast Guard that is above ensign and below lieutenant.
• noun One who holds this rank.

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

• noun A commissioned officer of the United States Navy whose rank is above an ensign's and below a lieutenant, or a corresponding rank in other navies.
• noun A commissioned officer in the United States Coast Guard, Public Health Service, or National Atmospheric and Oceanic Administration whose rank is above an ensign and below a lieutenant.

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

• noun an officer holding a commissioned rank in the United States Navy or United States Coast Guard; below lieutenant and above ensign