A supercookie is a Cookie with an origin of a top-level domain (such as .com) or a public suffix (such as .co.uk).
Ordinary cookies, by contrast, have an origin of a specific domain name, such as example.com.
Supercookies can be a potential security concern and are therefore often blocked by web browsers.
If unblocked by the browser, an attacker in control of a malicious website could set a supercookie and potentially disrupt or impersonate legitimate user requests to another website that shares the same top-level domain or public suffix as the malicious website.
For example, a supercookie with an origin of .com, could maliciously affect a request made to example.com, even if the cookie did not originate from example.com.
This can be used to fake logins or change user information.