Polygamy

Polygamy (from πολύς γάμος polys gamos, translated literally in Late Greek as "often married") is a form of marriage in which a person has more than one spouse at the same time, as opposed to monogamy in which a person has only one spouse at a time. When a man has more than one wife, the relationship is called polygyny; and when a woman has more than one husband, it is called polyandry. If a marriage includes multiple husbands and wives, it can be called group marriage.

The term is used in related ways in social anthropology, sociobiology, sociology, as well as in popular speech. In social anthropology, polygamy is the practice of a person's making him/herself available for two or more spouses to mate with. In contrast, monogamy is a marriage consisting of only two parties. Like monogamy, the term is often used in a de facto sense, applying regardless of whether the relationships are recognized by the state (see marriage for a discussion on the extent to which states can and do recognize potentially and actually polygamous forms as valid). In sociobiology, and zoology polygamy is used in a broad sense to mean any form of multiple mating.

It was legal in Utah for a while but was outlawed around the time it became a state.