let·ter·box 
(lĕtər-bŏks′)
(lĕtər-bŏks′)n.
1. See mailbox.
2. A format for presenting movies on television that maintains the rectangular theater image on a television screen by reducing the overall image until the full width can be seen, resulting in blank space above and below the image.
tr.v. let·ter·box·es, let·ter·box·ing, let·ter·boxed
To produce or present (film) in a letterbox format.