A cookie belonging to a different site from the page you're on. Safari and Firefox keep them from following you between sites; Chrome allows them.
Whether a cookie is first-party or third-party depends on the page, not on who set it. While the browser is on a network's tracking domain mid-redirect, the network's cookie is first-party. When the network's tag later loads on your order confirmation page, the same cookie is third-party, and that's where browsers differ.
- Safari blocks third-party cookies by default. Cookies a page's scripts write are deleted after seven days of browsing without a visit to the site, or capped at 24 hours on a page reached through a decorated link from a known tracker.
- Firefox has kept a separate cookie jar for each site since Total Cookie Protection became the default on 14 June 2022.
- Chrome still allows them by default. In April 2025 Google said it wouldn't introduce a new prompt, and in October 2025 it retired most Privacy Sandbox technologies.
This is why networks ask for their tag on your own pages and offer server-to-server tracking, where your systems report a sale with the network's click identifier taken from the landing address. Neither depends on a third-party cookie. A sale that shows no publisher when you expected one can come down to the browser.
In the UK, cookies for advertising and measurement need consent. Cookie window covers how long a referral lasts.
Google's own help and policy pages first.