access code

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

• noun An alphanumeric sequence that is entered on a keyboard or keypad and permits access to something, such as the controls of a device or a restricted location.

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

• noun an alphanumeric sequence that permits access to a secure service or network

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

• noun a code (a series of characters or digits) that must be entered in some way (typed or dialed or spoken) to get the use of something (a telephone line or a computer or a local area network etc.)