(informal) A platonic intimacy between two people (often a man and a woman) who work at the same workplace.

A form of polygamy formerly practiced by Mormons.

A form of polygynous, polyandrous marriage in which more than one man and more than one woman form a family unit.

Marriage between people of different socially-distinguished groups, such as different races, cultures or religions.

(US, informal) cohabitation, civil union / (US, informal) gay marriage

A marriage in which a widow marries her late husband's brother in order to maintain the family community.

plural of levirate marriage

A marriage in which a husband engages in marriage or sexual relations with the sister of his wife, usually after the death of his wife, or once his wife has proven infertile.

plural of sororate marriage

A marriage in which the partners decide not to have children, and to reserve the right to divorce amicably with mutual consent.