First-Party vs Third-Party Cookies: What's the Difference?

First-party cookies are set by the domain a visitor is actually on - used for things like keeping someone logged in, remembering a shopping cart, or a site's own analytics. Browsers generally still allow these without restriction, since they support the functioning of the site being visited.

Third-party cookies are set by a domain other than the one being visited - typically an ad network or tracking pixel embedded on the page - and are what enabled cross-site tracking: an ad platform recognising the same visitor across many unrelated websites. Major browsers have been restricting or blocking third-party cookies by default for several years now.

This distinction is why conversion tracking accuracy has been declining industry-wide and why solutions like server-side tagging and Enhanced Conversions exist - they shift tracking toward first-party data (a site's own domain, or hashed customer data sent directly) rather than relying on third-party cookies that an increasing share of browsers won't set in the first place. A tracking setup still assuming third-party cookies work reliably is measuring less of its actual traffic every year.