Vignetting

From Camerapedia.org

Vignetting is an optical effect in photography causing the corners of an image to be shaded, washed out or blurred - leaving a rounded picture. This can be due to poor lens design or construction, orsomething obstructing the lens, such as a too-small lens hood. It may also be done deliberately for effect - either using an add-on filter or hood on the lens, or in image processing software.