originally a picture of a unicorn posted on a blog site shortly after an offensive or disturbing picture has been posted, as a kind of antidote to the previous post. Now used of a comforting or innocent image, video clip, or topic viewed or contemplated following a previous, disturbing image, video, etc.
Word origin
originated by the blog site Boing Boing, which once posted a picture of a unicorn immediately after another editor had posteda photograph of his severely injured leg