open marriage

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

• noun A marriage in which both partners agree that each is free to engage in extramarital sexual relationships.

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

• noun A marital relationship in which each partner agrees that the other partner may have sexual relations with other people as he or she sees fit.

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

• noun a marriage in which each partner is free to enter into extraneous sexual relationships without guilt or jealousy from the other