Where a committed couple, usually married or in a licensed relationship agree to sexual involvement by one or both partners, with a third party.
When two people, who have a commitment to each other, agree jointly that one or both may seen sexual relations outside their particular agreement.
It does not imply that both will do so, merely that both agree, among themselves, to allow such occurrences, without repercussions, such as grounds for dissolving their own joint union.
The origins of the term/phrase are murky at best, however it appears that when it first appeared in the sixties (1960's) it referred to where people should be free to choose partners as they wished, rather than based on society standards, or values, or by religious restrictions.
Given the time, the meaning was about interracial marriages as much as it was about people being able to openly choose their partners, according to their feelings.
This was during the Civil Rights movement of that time as well.
The accepted definition seemed to have changed with the publication of the O'Neill book, in 1972, title Open Marriage.
It explored the concept that those involved in a relationship / marriage, were still entitled to grow as individuals, with outside influences, from a different circle of friends.
Primarily it dealt with issues of personal growth, as well as trust issues.
As well, it suggested that couples could grow or experience outside sexual relations, without harming the relationship, or limiting it.
Open Marriage is not Polygamy.
The main object, is to achieve personal growth, socially and sexually, for an individual. Ground Rules are usually a prerequisite for such a relationship, so that there are clear boundaries each are willing to grant the other member of the partnership.
In some instances, this refers to mainly sexual freedom to explore alternative sexual gratification, that the other may not be willing to commit to.
Some instances are where one has a larger sexual appetite, than the other, thus needing additional partners, for their sexual gratification.
It can also be about having different circles of friends, and does not diminish the value of each other's relationship.
Practice (Associated Acts):
In some areas, adultery is considered illegal, and even if the two parties agree to have sexual relations outside their formal bonds, it can still be a legal issue.
Studies generally tend to show that society does not approve of such a lifestyle, which makes success of such a relationship difficult to maintain.
It requires a form of secrecy that builds on jealousy feelings, which everyone admits to feeling, at one point or another. However, due to the disapproval of the majority, this can acerbate those feelings.
Researchers have estimated that between 1.7% and 6% of those married, are involved in some form of an Open Marriage.
They further claim that it hasn't appeared to have altered much, in terms of participating numbers.
Some of the more notable persons, who claim to be in an Open Marriage, are Shirley MacLaine (actress), Dolly Parton (singer, actress), Jean-Paul Sartre (author), Roger Stone (political strategist).