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.