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Third-party cookies let a tracking domain recognize users across unrelated websites. Safari and Firefox block them outright; Chrome has restricted them progressively. For affiliate attribution they are effectively dead technology.

Why the term still matters

Legacy tracking setups and older affiliate agreements still reference them, and understanding what broke explains today’s architecture: the industry’s shift to S2S postbacks, first-party data and server-side click storage is a direct response to their demise. If a vendor’s pitch still relies on third-party cookies, that’s a red flag about the whole product.