Closed Relationship / Closed Marriage / Closed Family
Agreement is made between all partners not to get romantically or sexually involved with individuals outside the relationship.


Friends With Benefits
Two or more people who get together for casual sex, who may or may not have some emotional attachment but are uncommitted to one another.


Group Relationship / Group Marriage
A group of three or more partners who agree to form a committed relationship.


Inclusive Relationship / Inclusive Family
A group relationship where partners may bring more people into the relationship.  The opposite of a closed relationship/ marriage/family.


Line Marriage / Line Relationship
A term originating from science-fiction author Robert A. Heinlein, used to describe a marriage or relationship where younger members are included over time to establish equalibriums as older spouses die off as a form of familial immortality.

Open Marriage / Open Relationship
A relationship where partners agree to having romantic or sexual relations with individuals outside the relationship.


PolyFamily
A relationship of three or more partners all living in the same residence.  Comparable to a traditional family, only with multiple spouses.


Polygamy / Polyandry / Polygyny
The practice of maintaining more than one partner.  "Polygyny" being the practice of a man taking multiple wives and "polyandry" being a woman with multiple husbands.  Usually refers to the practice of maintaining several marriage-like relationships with partners who generally do not share a sexual bond.


Quad
A relationship involving four partners. Often seen in the form of two couples who form a polyfamily.


Swinging  / Sport Sex / Swapping
A relationship where the partners agree to have casual sex with other couples or individuals. Swinging generally focuses on sexual gratification only,  rather than romantic or emotional involvement with people outside the relationship.

Triad
A relationship structure with three partners.


Vee
A triad where one partner is more  sexually or  emotionally  involved with the other two.  Example of this structure would be a straight man with two straight women.

 Zee
A relationship where four partners live in the same house, but only two are sexually involved with one another.  An example would be  two couples where one partner in each is disinterested or unable to have sex.