In human sexuality, a threesome is "a sexual interaction between three people whereby at least one engages in physical sexual behaviour with both the other individuals". While the term threesome typically refers to sexual activity involving three participants, it has at times been used to refer to a long-term domestic relationship, such as polyamory or a ménage à trois.

A ménage à trois is a domestic arrangement or committed relationship consisting of three people in polyamorous romantic or sexual relations with each other, and often dwelling together. The phrase is a loan from French meaning "household of three". Contemporary arrangements are sometimes identified as a throuple, thruple, or triad.