A gradual reduction in brightness from the centre to the corners of an image
When more light reaches the centre of the negative than reaches the periphery, the result is an image that is bright in the centre but dark in the corners: a vignette. Vignetting is inherent to most lenses (especially lenses with a wide angle-of-view). It becomes most apparent when the aperture is wide open, but will disappear when it is set a stop or two down from maximum.
The effect of this “light falloff” will largely go unnoticed, but is often visible when the subject being photographed has large solid areas of even colour and brightness.
In a very wide-angle lens, the falloff can leave the periphery two stops darker than the centre. This can be corrected with a graduated neutral density filter which is clear at the edges and darker at the centre, ensuring the frame is evenly illuminated.