Mirror cut-off is a term used in photography for a problem that can occur within a reflex camera. This occurs when the mirror in the camera is smaller than the focusing screen and using long (telephoto) lenses. When light exits a non-telephoto lens, the light beams diverge from the rear of the lens and for this reason the mirror can normally be a bit smaller (hence lighter & faster). However if the focal length of the lens is quite long what happens is that the light rays are near parallel and when they reflect off a smaller mirror, you see a cropped image in the viewfinder.
The aperture plays a part in the problem as well though when composing a shot the aperture is usually wide open and it is at the smaller apertures that mirror cut-off is most pronounced. When the actual picture is taken the sensor/film will see an uncropped image as the mirror is swung up out of the light path. This is a viewfinder problem but may affect composition.