trophy wife

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

• noun An attractive woman who is married to a usually much older, affluent man.

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

• noun A wife, usually young and attractive, regarded as a status symbol for the husband, usually older and affluent.

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

• noun a wife who is an attractive young woman; seldom the first wife of an affluent older man