Cookie ID

A cookie ID is a random value a site stores in the visitor's browser so that later requests from the same browser can be attributed to the same visitor.

An identifier the site writes on its own domain is first-party, while one written by a third domain is third-party; the second kind is now blocked in most browsers and is effectively gone for web measurement. Even a first-party identifier is not durable: browsers shorten the lifetime of cookies written from JavaScript, and the visitor can clear storage at any moment. The consequence shows up directly in reports, because every time the identifier is lost the same person is counted again as a new visitor and the return rate reads lower than reality. A cookie ID is the identity of a browser, not of a human being.