Keystone correction is a way to correct deformation (manifested as a trapezoidal shape) that occurs when images are projected (by projectors, etc.) onto a screen. It is thus referred to as "trapezoidal-deformation correction."
Trapezoidal deformation is generated when images are projected from a position above or below with respect to the plane of projection. Since it is an effective function in cases like when projectors are used in sites with restricted installation space, many projectors are fitted with a keystone-correction function.
