User ID
01 Definition
User ID meaning: A stable identifier you assign to a signed-in user to link activity across devices.
A User ID is a persistent, non-personal identifier you assign to a logged-in user and send to analytics. It lets Google Analytics connect that person's sessions across browsers and devices into one user. You must generate the ID yourself and avoid sending personally identifiable information in it.
Also called
Why it matters
How User ID fits the work
User ID enables cross-device analysis for signed-in audiences, giving a clearer view of the full customer journey than device-based tracking alone.
In context
When a customer signs in on both phone and laptop, the same User ID links those sessions, so GA4 reports them as one user.
Practical note
It only works when users log in, so anonymous activity still relies on client ID. Google's policies prohibit sending emails or other personal data as the User ID.