crown prince

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

• noun The male heir apparent to a throne.

from The Century Dictionary.

• noun The eldest son or other heir apparent of a monarch: applied more especially to German princes (translating German kronprinz).

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

• noun A person designated and raised to become the next king.
• noun A person designated and raised to become the next emperor.
• noun A person expected to inherit or take over an enterprise or undertaking once its current manager retires or dies.

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

• noun a male heir apparent to a throne