In Polyamory, which is defined as the state or practice of maintaining multiple sexual and/or romantic relationships simultaneously with the full knowledge and consent of all the people involved, the term Unicorn is used to describe a bisexual person (usually though not always female) who is willing to join an existing couple, often with the presumption that this person will date and become sexually involved with both members of that couple, and not demand anything or do anything which might cause problems or inconvenience to that couple.
The unicorn will be the girlfriend to the couple. The couple is usually considered a primary relationship, while the girlfriend will be a secondary partner to both. She isn’t allowed to do anything with one member of the triad, always with both. The dyad, on the other hand, are allowed to date each other without the girlfriend. If the U-Triad doesn’t work, then the dyad will stay together, and the girlfriend will leave. It’s understood that if things don’t work out that she will willingly leave with no issues. Though the dyad may choose to incorporate elements of the girlfriend’s life into the triad, it is not uncommon for the girlfriend to incorporate more of the dyad’s life into her own.
Sometimes the unicorn is expected to not develop any emotional attachment and is strictly there for a sexual relationship (equally distributed to both members of the dyad) and/or is prescripted as a secondary. This term is used as a reminder that bi poly women are people with their own desires, needs, and pre-existing lives, and not fantasy figures or pets.