Any material used to protect (from light, paint or whatever) all or part of an image, page or document. By extension, computer programs provide a feature which enables you to apply a virtual “mask” to all or selected parts of an image. Such masks are stored in an “alpha channel” and allow one to manipulate an image while leaving parts of it untouched. Masks have a wide range of uses, including hiding parts of an image without deleting them, making color and tonal adjustments to select parts of an image and blending images.