Unicorn, the white horse with the horn, has numerous alternative definitions, some of them not to be used in polite society.
I keep seeing the term used in financial documents, but with no explanation. Today I saw it defined, not once, but twice. Slashdot with Chase Purdy, "By contrast, Hampton Creek -- just a 20-mile drive from its Silicon Valley rival -- has raised more than $120 million since 2011. It's one of Silicon Valley's unicorns -- a company that has a valuation that exceeds $1 billion." Tech Crunch, "However, it’s last private valuation was close to $1 billion, the company priced under that mark, and has since moved past the $1 billion valuation threshold into unicorn-land."
So there we have it, apparently a unicorn, in financial parlance, is a company, possibly pre-IPO, that has a value greater than 1 billion.