Third-party cookie
A third-party cookie is set by a domain other than the one in the address bar, typically by an advertising network, a social media widget or an embedded video player, and lets that domain recognize the same browser across many websites.When a page loads a script, pixel or iframe from another domain, that domain can set and read its own cookies. Because the same advertising network is present on thousands of sites, its cookie follows the visitor from one to the next, which is the basis of cross-site tracking, retargeting and audience measurement across the web.
Browsers have been closing this channel. Safari blocks third-party cookies by default through Intelligent Tracking Prevention, Firefox through Enhanced Tracking Protection, and Brave by default. Google announced in July 2024 that Chrome would not retire third-party cookies after all, and confirmed in April 2025 that it would keep its current approach rather than add a new prompt. Third-party cookies therefore still work for most Chrome users.
Legally, third-party cookies almost always need prior consent in the EU, the UK, Türkiye and Brazil, because they serve advertising or social media purposes rather than a service the visitor asked for. In California and the other US states they are the typical example of a sale or sharing that visitors can opt out of. Embedded players and maps set them too, which is why a banner should hold YouTube and similar embeds until the visitor accepts the relevant category.