Unicorns have been known in the world for thousands of years back. They were mentioned in the bible, in mythology, and by great philosophers. They've played such a grand role in vast cultures, so What exactly is a unicorn?

"'Unicorn'. Old French, 'unicorne'. Latin, 'unicornis'. Literally, one horned: 'unus', one, and 'cornu', a horn. A fabulous animal resembling a horse with one horn. Visible only to those who search and trust, and generally mistaken for a white mare. Unicorn."
~Butterfly from "The Last Unicorn".

Unicorns are generally known as a white mare with one horn growing from the center of it's head, but it's also been seen as many other things: horned goats, bulls, deer, lions, etc. It usually has cloven hooves, and a spiralled horn.

In the bible they are a symbol of Christ (the horn symbolizes the Trinity of the Father, Son and Spirit). In ancient Babylon there were two main powers: the sun and the moon. The sun was represented by the lion, and the moon by the unicorn. The lion chased the unicorn, therefore representing the falling and rising of the sun and moon. I believe this old myth may still be regcocized today in the British Crest, from which this sily nursery-rhymish poem has derived:

The Lion and the Unicorn were fighting for the Crown:
The Lion chased the Unicorn all around the town.
Some gave them white bread and some gave them brown,
And some gave them plum cake and drummed them out of town.

Unicorns are generally known as having magical powers, usually held within their horns. For this power they are seeked by man. However, it is very hard indeed to catch a unicorn. They are very smart and quick, and they have thier magic to help them out. A book called the Physiologus was written in ancient times, and it's job was to name and define all creatures of the earth and their place within Creation. It said this about unicorns:

"He is a small animal, like a kid, but surprisingly fierce for his size, with one very sharp horn on his head, and no hunter is able to catch him by force. Yet there is a trick by which he is taken. Men lead a virgin to the place where he most resorts and leave her there alone. As soon as he sees this virgin he runs and lays his head in her lap. She fondles him and he falls asleep. The hunters then approach and capture him and lead him to the palace of the king."

Well, that's about all for now. If you'd like to find a more detailed, technical definition, I suggest you go to Unicorn Dream. I found most o my information there. It's an amazing site by a real unicorn expert if I've even met one. Check it out if you're interested. ^_^