Data
Third-Party Cookies
Small data files placed on users' browsers by domains other than the website being visited, historically used to track users across the web for ad targeting and attribution. Being phased out by browser restrictions (Safari and Firefox already block them, Chrome is implementing restrictions). Their deprecation has degraded retargeting accuracy, reduced attribution reliability, and increased the importance of first-party data and server-side tracking.
Why it matters
Their removal is the reason cross-site attribution degraded, and the reason models that were accurate in 2019 quietly stopped being accurate.
In practice
Assume any measurement built on them is understating. The question is not whether to migrate but what your current numbers are already missing.
Related terms