Originally Posted by Vallin
What is the exact definition of "unicorn"?
A unicorn in polyamorous circles colloquially refers to a single, bisexual woman, who is interested in forming a polyfidelitous triad with an already established couple of a man and a woman. The choice of the label unicorn should demonstrate the perceived rarity of such individuals and the somewhat belittling and diminutive fashion that people looking for them are often viewed in by the polyamorous community.
If you need any part of this explanation explained I suggest you do a little research instead of asking. You could start by rereading this thread more carefully because all of this could have been fairly well inferred, even though it was not explicitly stated.
If this sounds like it has a tone. Sorry, it's only somewhat unintentional.