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.