closed loop relationship
An intimate relationship of common understanding and agreement among three individuals, usually where two persons are married or similarly bound; where individuals may be heterosexual, homosexual, or bisexual; and, where all individuals remain faithful and exclusive to each other.
Unlike a ménage à trois, a closed loop relationship ("clr") is typically a long-term relationship that involves a third individual as a functioning, participating partner. Also unlike a ménage à trois, intimacy is not the sole motivation and serves as complement to the relationship. Individuals in a clr may live together, though this is not typical. At the fringes of a clr is the situation where one participant is a committed participant in another relationship, such as being married.