Dating, Decoded:

Unicorn

What does unicorn mean in non-monogamy? The term explained.

In non-monogamy, a "unicorn" refers to a single person — often assumed to be bisexual and unattached — sought out by an existing couple to join them, usually for a casual or occasional dynamic. The term gets a mixed reputation because "unicorn hunting" can mean a couple treating that person as an accessory rather than an equal partner. If you're the one being sought after, make sure you're treated as a full person in the dynamic, not a shared toy.

See: Ethical Non-Monogamy (ENM)