Third-party cookies

A third-party cookie is a cookie set by a domain other than the one shown in the browser's address bar — typically an advertising, analytics or social widget embedded on the page. Because the same ad-tech domain can be embedded across thousands of unrelated sites, it can recognize the same browser across all of them, which is what made third-party cookies the technical backbone of cross-site ad tracking and retargeting for two decades.

This differs from a first-party cookie, set by the site a visitor is actually on and readable only by that site, which is far less useful for tracking someone across the web but is generally treated as lower-risk and is not the target of most current browser restrictions. Safari and Firefox block third-party cookies by default, and Chrome has moved to restrict them as well, which is why the industry has shifted toward cookieless tracking approaches, server-side tagging, and building owned first-party data.

The common confusion is treating "cookies are being blocked" as a blanket statement: the restrictions target third-party cookies specifically, so a site's own first-party analytics generally still works, though signal loss still occurs indirectly wherever a site relied on a third-party service to stitch identity across domains or devices.