Field of view
Field of view or FOV is the extent of observable world at any given moment. Field of view is usually measured in degrees. Humans have 180 degrees FOV when looking directly in front and 270 degrees with eye rotation. The higher an HMD's FOV is, the further the virtual world will extend to your edge of vision. You generally want an HMD with at least 90 degrees FOV.
In VR, there are two types of FOVs: Display FOV or dFOV and Camera FOV or cFOV.
Display FOV or dFOV is the FOV of your HMD.
While wide dFOV can increase immersion and induce presence, it can cause simulator sickness in certain individuals. Large dFOV can cause discomfort due to 2 reasons. First, humans are more sensible to the flickers and movements of images in the peripheries of their visual systems. Second, large dFOV means provides more overall visual input when compared to small dFOV. Too much visual input can cause conflicts with vestibular and proprioceptive system.
Camera FOV or cFOV is the FOV of the virtual environment rendered by the graphical engine.
Changing cFOV can lead to unnatural movement of the virtual environment in response to head movements. For example moving your head for 7 degrees in real time creates 11 degrees of movement in VR. This can lead to discomfort and a maladaptive condition called vestibular-ocular reflex gain adaptation.
Using Subtle Dynamic FOV to Combat Simulator Sickness
Researchers from Columbia Engineering have used to changes in FOV to combat Simulator sickness. Researchers subtly decreased the users' FOV while they are moving in VR. They restored the users' FOV while they are standing still in VR. Researchers found that they were able to reduce discomfort while maintaining presence. 
- ↑ 1.0 1.1 https://developer.oculus.com/documentation/intro-vr/latest/concepts/bp_app_simulator_sickness/
- ↑ http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=7460053