Glossary
What is Third-party cookie?
Also called: 3P cookie, cross-site cookie
A third-party cookie is set by a domain other than the site the reader is on, such as an ad company's, letting it recognise the same browser across sites.
हिंदी में पढ़ेंA first-party cookie comes from the site itself, for example to remember a login. A third-party cookie is set by another domain on the page, such as an ad company. When the reader later visits a different site where that company is present, it can read the same cookie again. That is how buyers have recognised one browser across sites, for targeting, frequency capping and measurement.
Browsers differ. Safari has blocked cookies for cross-site resources by default since 2020, and Firefox’s Total Cookie Protection gives each site its own cookie jar. Chrome announced a phase-out in 2020, switched to a user-choice plan in July 2024, dropped the planned standalone cookie prompt in April 2025 and retired most Privacy Sandbox replacements in October 2025. So in 2026 Chrome still allows third-party cookies by default; users can block them in settings, and Incognito blocks them.
Identity still moves prices. In a 2019 test on its own stack, Google found publisher revenue fell by about half when third-party cookies were disabled for a slice of traffic. Publishers cover Safari and Firefox with first-party IDs, logged-in IDs and contextual signals.
Example
A site's traffic is 60% Chrome, 30% Safari and 10% Firefox. Cross-site recognition through third-party cookies works by default only on the 60% Chrome share. On the other 40%, buyers need first-party IDs or page context.
Why it matters to publishers
Readers that buyers recognise usually attract higher bids, so your browser mix affects CPM. Splitting CPM by browser shows what identity is worth on your site.