Device ID

A device ID is an identifier that stands for one device or browser rather than one person, and it is the default identity for a visitor who has not signed in.

On the web the name is slightly misleading, because what is really counted is a browser: two browsers on one laptop produce two separate identifiers, and one shared home laptop collapses several people under a single identifier. In mobile SDKs the identifier is closer to a real device, but it is regenerated when the application is reinstalled. Keep three identifiers apart: the device identifier is a browser identity, the user identifier is an account identity, and the resolved identifier is what the system uses for reporting after choosing between the two. Reporting users on a device-identifier basis and calling them people is the most widespread overstatement in web measurement.