Ménage À Trois
Pronunciation:

[māˈnäZH ä ˈt(r)wä]
Definition
A sexual arrangement involving exactly three members. Any variation of gender and sexuality is acceptable, but it must include three members partaking in sexual activity with one another to be considered a true ménage à trois.
A typical example of a ménage à trois is when a married couple asks someone to join in on love making session.
Any grouping which involves more than three people OR has members that are not engaging in direct sexual activity with all of the other members is either group sex or a gang bang.
Example
Helena and Tina were discussing their various sexual adventures over coffee. Having both been married for a while, they relished the opportunity to share stories. Helena: “Mike and I invited Sharon over for a ménage à trois the other night, it was so fun!”