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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.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(of a video recording of a movie) formatted so as to retain the aspect ratio of the original cinematic release
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014