chief executive

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

• noun The President of the United States.
• noun A principal executive official, such as the leader of a nation's government or the governor of a state.

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

• noun The president of the United States.
• noun The head of government or head of state.
• noun The governor of a U.S. state.
• noun chief executive officer

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

• noun the person who holds the office of head of state of the United States government
• noun the office of the United States head of state