Digital Lexicon Hub

A cookie set by a domain other than the site the user is visiting, usually through embedded scripts, iframes, pixels or images.

Retargeting, cross-site frequency capping and view-through attribution have relied on third-party cookies, so browser restrictions reduce audience size and measured conversions and push budgets toward first-party data and modeled reporting.

A B2B advertiser sees 35% of site visitors on Safari and Firefox drop out of its retargeting pool. It adds Conversions API and Enhanced Conversions, grows logged-in content, and uploads hashed CRM emails as audiences instead of relying on pixels alone.

Chrome still allows third-party cookies by default and kept user choice when Google said in October 2025 it would retire most Privacy Sandbox APIs, so the impact varies by browser mix. Consent rules under the ePrivacy Directive and GDPR apply regardless of browser support.

A cookie is third-party when its domain and scheme differ from the top-level site in the address bar. To be sent in cross-site requests it needs SameSite=None together with the Secure attribute. Safari blocks them through Intelligent Tracking Prevention and Firefox isolates them per site with Total Cookie Protection. CHIPS lets a cookie use the Partitioned attribute so it works only within one top-level site, which limits Cross-Device Tracking and shifts measurement to First-Party Data.