Letterbox

A video format that allows a video with a different aspect ratio to be displayed on a display that does not support that aspect ratio. This is commonly done with DVD videos today to allow the user to view the movie as it was originally intended to be viewed and not modified to fit your television screen. A DVD that has this type of display will mention it's viewed in widescreen format. Some DVDs support both widescreen and full screen format, allowing the viewer to decide how they want to view it. Unless the DVD supports full screen format, it will only be viewable in widescreen.
Below is an example of what a letterbox may look like on a display. As can be seen below, letterbox is easily distinguishable by the horizontal black bars along the bottom and top of the image.

Also see: Aspect ratio, CD definitions, DVD, Video definitions, Widescreen