Field Of View (FOV) is a traditional photography term, meaning the area that the camera can see in the game scene.
The FOV is usually defined by its width in degrees. A typical FOV in games is around 60 degrees, although players enjoy being able to change the FOV themselves.
When linear perspective is used, this produces distortions in the scene where objects in the center of the view appear to be farther away than they are, and objects at the periphery appear to be really close.
A really wide FOV can lower the frame rate because so many more polygons are visible.
Fisheye Quake allows an extremely wide FOV while avoiding the typical linear distortion artifacts, by rendering the game as a cubemap, then stitching it together into an anamorphic projection.