Open Marriage

A term that is often used to describe a marriage relationship in which the husband and wife have no reservations about being sexually and/or emotionally involved with other people. Couples involved in open marriages typically adopt a set of ground rules to guide their activities.

The 1972 book, Open Marriage (by George and Nena O'Neill) is often considered the source for the term, although the authors only briefly touched upon the subject extramarital sex within marriage.