unicorn (_ju_n_k__n/)noun: unicorn; plural noun: unicorns 1. a mythical animal typically represented as a horse with a single straight horn projecting from its forehead. a heraldic representation of a unicorn, with a twisted horn, a deer's feet, a goat's beard, and a lion's tail. 2. a start-up company valued at more than a billion dollars, typically in the software or technology sector.