A mythical creature resembling a horse, with a single horn in the center of its forehead: often symbolic of chastity or purity.
We (the Royal we, being my friends and I) also refer to The Unicorn as a curse. (N.B. If you are one of my friends and didn't know about this, take notes) For us, The Unicorn is also known as "The One Who Got Away". Love perfected. The ideal partner. The woulda/coulda/shoulda relationship we have all had at some point our lives.
More sacrifices have been made in the name of The Unicorn than we care to count. Stop and think about it for a second. Is there one person in your life that you wish you could go back to? Be with? Track down? That one person, who, if they called, you would stop what you were doing to see if it would work out? The standard who you compare all your relationships againt? That's The Unicorn. I've had a few.
Unicorns are bullshit. They don't really exist. What you see, at a distance, is a big white horse with an optical illusion sticking out of its forehead. You get up close to it, give it some time, and you will find that the horn isn't a horn, the hooves aren't really silver (or cloven, or ring like bells, or whatever version you want) And if you get to close, you are either going to get covered in horseshit or trampled. Maybe both. (My unicorn turned out to be an ass.)
The only way to learn that The Unicorn is really a horse is to chase it. People can talk until they are blue in the face, all you will hear is blah blah blah. Funny thing, though. Sometimes, just sometimes, you get up close and it turns out that you were looking for a horse in the first place, and things work out fine. Your unicorn is taken from myth to reality, and it's a reality you can live with.
As for me, while I love the idea of unicorns, I just don't believe in the myth anymore.