Vignette

https://docs.unity3d.com/Manual/PostProcessing-Vignette.html

 

The effect descriptions on this page refer to the default effects found within the post-processing stack.

In Photography, vignetting is the term used for the darkening and/or desaturating towards the edges of an image compared to the center. This is usually caused by thick or stacked filters, secondary lenses, and improper lens hoods. It is also often used for artistic effect, such as to draw focus to the center of an image.