Evercookie
en.wikipedia.org/wiki/Evercookie
Article
Evercookie
JavaScript application programming interface
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Evercookie is not merely difficult to delete, it actively "resists" deletion by redundantly copying itself in different forms on the user's machine, and resurrecting itself if it notices some of the copies are missing or expired. As such, it serves to highlight the ways in which creators of malware can attack browsers.