visitor ID

A usage analytics visitor ID uniquely identifies a user in a search interface, whether they’re authenticated or not. It can be manually specified or automatically generated and it never expires, so it can be used to track individual users across multiple visits.

The usage analytics visitor ID is distinct from the visit ID, which does expire, and the user ID, which is automatically attributed and is used to track authenticated users. It’s also distinct from the client ID, which takes precendence over the visitor ID when both are present (see Client ID, Visitor ID, and Visit ID Dimensions).