Open marriage refers to a marriage where both spouses agree to allow sexual relationships for one or both outside the marriage, without considering this to be sexual infidelity. It is distinguished from "swinging" in that it often involves activities separate from the spouse. The rules or boundaries in an open marriage vary by couple.
Extra-marital sex is often illegal in jurisdictions where adultery is illegal, regardless of whether the partner(s) have given their consent. Open marriage differs from polygamy, where sexual relationships exist only among the parties to the marriage.