heir apparent

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

• noun An heir who will inherit the ancestor's estate, title, or office as long as the ancestor does not exclude the heir in any will and the heir survives the ancestor.

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

• noun Someone who will definitely inherit if surviving the one whose property is to be inherited.

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

• noun an heir whose right to an inheritance cannot be defeated if that person outlives the ancestor