lieutenant colonel

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

• noun A commissioned rank in the US Army, Air Force, or Marine Corps that is above major and below colonel.
• noun One who holds this rank.

from The Century Dictionary.

• noun A military officer next in rank below a colonel, and in some European armies commonly the actual commander of a regiment, the colonelship being honorary.

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

• noun An army officer in most countries, ranking below a colonel and above a major. Often the commander of a battalion or the executive officer of a regiment.
• noun An officer in the United States Marine Corps or Royal Marines, of equivalent rank to their counterparts in the army.
• noun An officer in the United States Air Force, Air Force Reserve, or Air National Guard, ranking below colonel and above major. Often the commander of a squadron or the executive officer of a wing.

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

• noun a commissioned officer in the United States Army or Air Force or Marines holding a rank above major and below colonel