Supercookies, also known as evercookies or zombie cookies, are like browser cookies in that they can be used to track you, but are much harder to remove.
What Is a Supercookie?
The way I first heard supercookies described was as a cookie that you can appear to delete, but as soon as you do, software rewrites the cookie. Like the Hydra from Greek mythology, cutting off a head does no good because it grows back [1].