The field of vision represents a field of the observable world that is being observed at the exact moment.
In first person shooters and other 3D games, FOV is frequently an ingame variable that can be adjusted, though a more accurate term to describe its function would be angle of view. As an example, in a first person shooter, the FOV variable directly corrospond's to the angle of the protagonist's cone of vision. The wider the angle, the wider the cone is, and therefore, a larger portion of the game world is visible on the screen.
For some games, the horizontal component of the angle of view is directly controlled by the FOV variable, and the vertical component is automatically adjusted based on the screen's aspect ratio. For some others, both the horizontal component and the vertical component are directly controlled by the same FOV variable, causing proportions to become incorrect on displays with wide aspect ratios. Very few games have separate variables to controll the horizontal and vertical component.