Keystoning
From The Glossary of Digital Photography
When a projector is not perpendicular to the screen, a distortion in the projected image occurs where the rectangular image is not projected as rectangular on the screen. Sharpness is also reduced on the affected edges. This can be adjusted by making sure that the projector is placed so that it is square to the screen. This generally means that it is at the center of the screen in height. With the advent of digital projectors, adjustments can usually be made electronically to digitally distort the image vertically before it leaves the projector so that it appears rectangular on the screen.
