What is the "Linear" field of view?

Q: What is the "Linear" field of view?

A: This is a new field of view that is slightly cropped in (similar to "Medium") that improves any unwanted distortion on horizons (as usually found on "traditional" GoPro Images) and gives a much more natural look to the frame. 

In the Session camera the feature is available in Video Mode and in the Black camera it is available in photo Stills and Video modes. 

The feature is not available in 4K Video and some advanced photo modes. For example please see below image courtesy of Have Camera Will Travel Blog: