A unicorn is a rare person who appears to be the perfect catch. Because this type of person is hard to find or hard to catch, they are akin to the rare, mythical unicorn.
The unicorn term originated in the polyamorous and swinging communities but it has become more mainstream over time. In 2019, the term became more well-known with the release of a TV sitcom called The Unicorn, about a widowed father of two coming to terms with his newfound desirability. In this show, the character is a unicorn because he is a good single father with no commitment issues.
In polyamory or swinging lifestyles, a unicorn is a desirable person willing to have sex or enter a relationship with an established couple. In this context, the term unicorn usually refers to a single, bisexual female. However, it can also apply to people of any gender or sexual identity.
For swingers, a unicorn is someone who will have sex with both partners with no strings or drama attached. In the polyamorous community, a unicorn may be expected to be a part of a closed triad. This means they will not date anyone outside of the three-person relationship. People looking for these unicorns are known as unicorn hunters.
What constitutes a unicorn varies in different circles. A unicorn can theoretically be someone of any gender or sexual identity. However, when someone calls someone a unicorn, they refer to the groups of people to which they are personally attracted.