a marriage in which the partners are free to have sexual relationships with other people.
We have an open marriage, but I never tell my husband about my other lovers.
an open marriage
A marriage between two people who do not object to each other continuing to have other sexual or romantic partners. A: "Lila has a new boyfriend? I thought she was married to Chad." B: "Oh, they have an open marriage."Don't worry, I still date other people—my wife and I have an open marriage.