A unique, resettable identifier assigned by Apple to iOS devices for advertising measurement and audience targeting, subject to user opt-in controls under App Tracking Transparency.
Since Apple’s App Tracking Transparency framework made IDFA opt-in, a large share of iOS users don’t share it at all which has pushed advertisers toward first-party data, contextual targeting, and probabilistic modeling to fill the measurement gap that IDFA used to cover.
Further reading: Identity layer · Identity after third-party cookies
