skeleton key

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

• noun A key with a large portion of the bit filed away so that it can open different locks.

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

• noun A very simple design of key that usually has a cylindrical shaft (sometimes called a "shank") and a single, minimal flat, rectangular tooth or "bit".
• noun A key that has parts filed away so that it will open a range of locks

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

• noun a passkey with much of the bit filed away so that it can open different locks