Unicorn

A single, usually bisexual woman willing to join an existing couple, so called because she is considered rare.

A unicorn is the lifestyle term for a single, often bisexual woman who is open to joining an established couple, either for play or something ongoing. The name reflects how sought after and relatively rare such an arrangement is. The community sometimes cautions couples against unicorn hunting in a way that treats a person as an accessory, and encourages treating any third person as an equal with their own needs and boundaries. Respect and honesty matter as much here as anywhere.