heir presumptive

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

• noun An heir who stands to inherit the estate, title, or office of an intestate ancestor, but whose claim can be defeated if a relative who is closer to the ancestor is born prior to the ancestor's death.

from The Century Dictionary.

• noun See heir apparent.

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

• noun Someone who will inherit only if no better choice for an heir is born.

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

• noun a person who expects to inherit but whose right can be defeated by the birth of a nearer relative