Vignette
Darkening the image toward its corners — in real lenses an optical defect, in games a deliberate device to pull the eye inward.
Real lenses pass less light at the edges than in the centre, partly for geometric reasons (the so-called cos⁴ law) and partly because the lens barrel or hood shades the rays at an angle. Photographers have forever both fought and cultivated the effect, since a darkened edge frames the subject and lends an older, more filmic feel.
In real-time graphics it is one of the cheapest post-processing steps: a radial gradient multiplied over the whole image after the scene is drawn. Games use it to draw the eye toward the centre, heighten mood in horror and dark passages, or frame a HUD. It is often combined with tone mapping and color grading in the same pass.