An open relationship denotes a relationship (usually between two people) in which participants are free to take other partners; if the couple making this agreement are married, it is an open marriage. While ‘open relationship’ is sometimes used as a synonym to ‘polyamory‘ or ‘polyamorous relationship’, these terms are generally differentiated with the “open” in ‘open relationship’ usually refers to the sexual aspect of a non-closed relationship, whereas polyamory refers to the extension of a relationship by allowing bonds to form (which may be sexual or otherwise) as additional long term relationships:
- Some relationships place strict restrictions on partners (e.g. polyfidelity); such relationships are polyamorous, but not open.
- Some relationships permit sex outside the primary relationship, but not love (cf swinging); such relationships are open, but not polyamorous.
- Some polyamorists do not accept the dichotomies of “in a relationship/not in a relationship” and “partners/not partners”; without these divisions, it is meaningless to class a relationship as ‘open’ and ‘closed’.
- Some polyamorists consider ‘polyamory’ to be their philosophical orientation they believe themselves capable and desirous of multiple loves whereas ‘open relationship’ is used as a logistical description: that is, it is how their polyamory is expressed or implemented. They would say of themselves, for instance, “I am polyamorous; my primary partner and I have an open relationship (with the following ground rules)….”
However, there is enough overlap between the two concepts that ‘open relationship’ is sometimes used as a catch-all substitute when speaking to people who may not be familiar with ‘polyamory’.
Source2: http://en.wikipedia.org/wiki/Open_relationship )Open Marriage
The origins of the term open marriage remain obscure. Researchers in the 1960s used the term open marriage to describe individual freedom in choosing marriage partners.   Closed marriage meant individuals had to marry someone based on social prohibitions and social prescriptions. Open marriage meant individuals could choose to marry someone based on personal preferences. Nena O’Neill and George O’Neill changed the meaning of the term with the 1972 publication of their book Open Marriage, which sold over 1.5 million copies. The O’Neills conceived open marriage as one that gives each partner room for personal growth and allows each partner to develop outside friendships. Most chapters in the book dealt with non-controversial approaches to revitalizing marriage in areas of trust, role flexibility, communication, identity, and equality. Chapter 16, entitled “Love Without Jealousy,” devoted 20 pages to the proposition that an open marriage could include some forms of sexuality with other partners. The concepts of this chapter took root in the cultural consciousness. Popular culture began using the term open marriage as a synonym for sexually non-monogamous marriage, much to the regret of the O’Neills. In the 1977 publication of The Marriage Premise, Nena O’Neill advocated sexual fidelity in the chapter of that name. By then however, the concept of open marriage as sexually non-monogamous marriage had gained a life of its own.
Today, with many committed couples not seeking formal marriage, the term is frequently generalized to open relationship or responsible non-monogamy. The concept of being sexually open versus closed is also sometimes applied to triads or other groups larger than two.