What is a ménage à trois?
A ménage à trois is a French phrase — literally 'household of three' — for a sexual arrangement involving three people, commonly a threesome. In the lifestyle it usually means a consensual threesome, often a couple plus a third (a 'unicorn' when that third is a single woman), with everyone's enthusiastic agreement.
Ménage à trois is a French term that translates literally as 'household of three.' Historically it described a domestic arrangement where three people lived together in a relationship; in everyday modern use it most often means a threesome — three people involved sexually at once.
In the lifestyle, a ménage à trois is simply a consensual threesome. A very common form is a committed couple inviting a third person to join them. When that third is a single woman who enjoys being with couples, she is often called a 'unicorn.' Configurations are usually described with shorthand like MFM (two men, one woman) or FMF (two women, one man).
As with any lifestyle activity, the foundation is enthusiastic consent from all three people, clear communication about boundaries beforehand, and respect — the third person is an equal participant, not an accessory, and the couple's own relationship is discussed and protected.
For the related terms and dynamics, see the JoinTheSwing glossary and the answers on unicorns, throuples, and the lifestyle generally.