Anonymous ID
Beginner PronunciationEN/əˈnɑːnɪməs ˌaɪ ˈdiː/
An anonymous ID is the identifier an analytics library generates and stores on a device the first time it sees a visitor, before any login happens.
The value lives in a first-party cookie or in browser local storage and rides along with every event, so a visitor's actions hang together before any account exists. Its limit is that it identifies a browser and not a person: a new browser, a second device, a private window or a cleared cookie each produce a fresh value, which inflates user counts. The name is also misleading. The value is random, but because it is persistent and tied to behaviour, most privacy regimes treat it as personal data rather than as anonymous data.
FAQ
The anonymous ID is created by the analytics library and belongs to the device; the user ID comes from your own system and belongs to the account. Both can sit on the same event, and wherever the user ID is present it is the authoritative one.