system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 what exactly is Verizon's and AT&T's "perma-cookies"?
gpt
Well a 'cookie' is a piece of data that is typically stored on a user's computer/device (via their web browser) and used to identify that user  (e.g. for purposes of authenticating the user to a website).

A 'perma-cookie' usually just refers to a cookie that is quite persistent and hard to remove. It may, for example, be used to identify the user even after clearing their cookies in their web browser's settings/configuration menu.

In the case of some mobile carriers like Verizon, a so-called 'perma-cookie' is being injected into user's web traffic. This effectively may allow any website you visit to uniquely identify you and cross-reference your activity on that site with your activity on other websites you visit. The thing about these perma-cookies is that they are not stored on the web browser / device, instead the carrier is injecting them midstream between you and the web-server. This means that it's impossible for you to disable / remove the 'perma-cookie' to prevent you from being tracked across websites.