An user is defined as a person or device that has had at least one session with your app during a given time period.

If a user launches more than one session during a given period, they will only be counted once.

For example a user of an application (app) on a mobile device. A user is first counted when launching an app. Launching an app is the first event of a session. 
When the user returns (launches again), she/ he becomes a returning user.
Every user is counted only once, based on a unique combination of measurements.

A user is the same as a unique (website) visitor.

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