Privacy
Third-party cookie
Also called 3P cookie.
Third-party cookie is a cookie set by a host other than the page the person is viewing. Ad networks, embedded players, and social buttons are the usual sources.
How it is measured
Compare the cookie's domain with the page. A cookie from adnet.example on shop.example is third party. Safari and Firefox block most by default, and Chrome has kept them available after repeated delays.
Count them per page and per outside domain. The number of distinct outside domains is the real measure of exposure.
Worked example
A recipe blog embeds a video player and an ad slot. The storage panel shows 7 cookies from 3 outside domains in Chrome. In Safari, none arrive.
Ad revenue per thousand views is 40 percent lower in Safari, because the ad network cannot recognize the visitor.
How it differs
A third-party cookie comes from another host. A first-party cookie comes from the visited site. The third excludes the site's control over it; the first excludes reach across other sites.
Common errors
Assuming Chrome has removed them. Planning around one browser. Ignoring consent. Believing blockers catch them all. Mixing them up with third-party scripts that set first-party cookies.
In practice
Count outside domains on your top three pages. Remove embeds that earn little. Do not build a measurement plan that depends on them.