A defect in an optical system that makes the edges of the image darker than the center.
Most cheap lenses in single-use cameras vignette. Luckily the human vision system seems to be able to filter vignetting quite easily.
Very frequent in wideangles: it is fixed by redesigning the lens, when possible, or adding what is called a center filter, that's to say a piece of transparent material with a darker area in the centre.

In a centered optical system, vignetting occurs when the entrance window lets in light rays that can't reach the center of the entrance pupil. This results in a penumbra zone around the image. To get rid of vignetting, one must design the system so that the entrance window is in the object plane and the exit window is in the image plane.