Multilateral marriage is a term for a relationship where three or more people consider themselves to be married to the others in the relationship group. Group marriage researchers Larry and Joan Constantine coined the term in their 1973 book Group Marriage: A Study of Contemporary Multilateral Marriage.
Today, multilateral marriage is considered a type of group marriage. However, the Constantines created the term as an alternative to group marriage, which at the time referred to a four-person marriage between two men and two women. The Constantines’ idea of multilateral marriage could include a marriage group of any size, made up of people of any gender and sexual orientation. They dubbed the people in these types of marriages cohusbands and cowives.