Light fall-off in the corners of images due to lens characteristics. There are mainly two types of vignetting. One caused by the lens housing sometimes called artificial vignetting and one caused by optical reasons (cosine-in-4th-power law) sometimes called natural vignetting. Only the last one can be easily corrected by the Panorama Tools Plugins radial luminance filter. In principle Fulvio Senore's program for vignetting correction can correct both types of vignetting.
Although the Panorama Tools Plugins radial luminance filter implements vignetting correction, this functionality was never added to PTStitcher. In this case it is necessary to pre-process photographs with the plugin or some other tool.