lieutenant commander

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 lieutenant and below commander.
• noun One who holds this rank.

from The Century Dictionary.

• noun A commissioned officer in the United States navy, of a grade intermediate between that of commander and that of lieutenant, and ranking with a major in the army.

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

• noun A naval officer whose rank is higher than a lieutenant and below that of a commander.

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

• noun a commissioned officer in the Navy ranking above a lieutenant and below a commander