Definitions for "Vignetting"
The gradual reduction of image illuminance with an increasing off-axis angle, resulting from limitations of the clear apertures of elements within an optical system.
An unintentional, shaded loss of the edges of an image or picture by an optical component clipping the peripheral beams can lead to loss of contrast in video.
A fall-off in brightness at the edges of an image, slide, or print.
Keywords:  unexposed, oval, enlarger, border, left
Blocking the light from an enlarger so as to leave a portion of the border unexposed. The area left unexposed (and therefore white) is usually oval in shape.
The action of blurring the background so that the subject stands out.