FOV, an abbreviation for field of view or field of vision, describes to what degree the player is able to see his surroundings. FOV is typically expressed in degrees, specifically how wide the player's peripheral vision extends. By default, FOV is set to 90 degrees, and can be changed at will using the console command fov [angle].
Some players will use alias commands in conjunction with FOV to create a simple sniper zoom effect, by reducing the FOV to a lower number, thus causing the view to appear zoomed in.
It is possible from the gameplay options menu to disable changing the FOV.