The definition of 'roll' in android is:

> 
> angle between a plane perpendicular to the device's screen and a plane perpendicular to the ground
> 
> 
> 

See [here](https://developer.android.com/reference/android/hardware/SensorManager.html#getOrientation(float%5B%5D,%20float%5B%5D)). 

Maybe I'm missing something, but there seem to be many planes perpendicular to the ground giving many different angles.

Could someone please rephrase this definition using e.g planes parallel to ground or screen, north direction, gravity direction?

NB! I am asking for a rephrasing using different words. I know the official definition as I have stated. Please don't repost the official definition.