Advertising ID
Also known as: idfa, gaid, mobile ad id
A resettable per-device identifier iOS and Android give to apps for ad tracking.
Apple's IDFA and Google's GAID are unique strings assigned to each phone so ad networks can attribute installs and conversions across apps without needing cookies. Users can reset or, on iOS since ATT, refuse to share them entirely.
When the ad ID is unavailable, ad networks fall back to fingerprinting (device model, IP, screen) and probabilistic matching. Refusing the ad ID reduces but doesn't eliminate cross-app tracking.
Related terms
Code, pixel or request that collects information about a user across websites.
Identifying a user by combining many small browser and device attributes into a unique signature.
The iOS prompt that requires apps to ask permission before tracking you across other apps and sites.