The definition of a 'Unicorn' in this instance is a single women who is prepared to 'play' and or live with a couple.
The term 'unicorn' is attached as they are rare or mythical creatures that don't exist. I was one once until I met my partner and soul mate. Considering we are rare and mythical we are in great demand!
A Unicorn can live out every fantasy she wishes as the opportunity and the offers are always there. A true Unicorn knows herself, is comfortable in her own skin and most importantly will not consent to any situation she considers to be detrimental to one of the parties involved. She posses great natural empathy.
Yes a true Unicorn is a rare and precious find.