any of the three-digit telephone codes assigned to each of the areas into which the U.S., Canada, etc. are divided
See area code in American Heritage Dictionary 4
A three-digit number 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.
A similar number of varying length assigned to geographic areas in other parts of the world.