Third-party cookies are cookies set by a domain other than the one the user is currently visiting — typically by advertising networks, analytics providers, or social media platforms whose scripts are embedded on many sites. They enable cross-site tracking, allowing advertisers to follow users across the web and build behavioural profiles for ad targeting. Firefox and Safari blocked third-party cookies by default years ago; Google announced plans to do the same in Chrome but repeatedly delayed the deprecation timeline, ultimately deciding in 2024 not to deprecate them wholesale but instead to introduce user-level privacy controls. The uncertainty has driven adoption of alternative measurement approaches including server-side tracking, first-party data strategies, and the Privacy Sandbox APIs. Third-party cookie loss primarily affects paid media attribution and remarketing rather than organic SEO directly.