Vignetting
A darkening at the edges of the sight picture, usually caused by short eye relief or an off-center eye blocking part of the light cone.
Vignetting is the gradual loss of brightness toward the outer edges of the image, which leaves the center bright while the corners fade into shadow. Behind a rifle scope it most often appears when the eye sits too close, too far, or off to one side, so part of the light cone leaving the eyepiece never reaches the pupil. Because the visible light cone is defined by the exit pupil, an eye placed outside that cone clips the edges of the picture.
Correct head position fixes most of it: settle into the right eye relief distance and center the eye, and the full bright sight picture returns. When the darkening grows into a distinct dark ring or crescent, shooters usually call it scope shadow instead. A little vignetting is normal in any optic, but heavy edge darkening on a rifle scope is a clear cue to fix the mount or the cheek weld.