area code

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

• noun A three-digit number that is assigned to a geographical area in the United States, Canada, and the Caribbean, used before a seven-digit telephone number when placing a call to, and sometimes from within, that area.
• noun A similar number of varying length assigned to geographic areas in other parts of the world.

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

• noun A prefix appended to a telephone number indicating the geographical area.

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

• noun a number usually of 3 digits assigned to a telephone area as in the United States and Canada