Collins COBUILD Advanced Learner's English Dictionary - letterbox

also letter box (letterboxes) 1. A letterbox is a rectangular hole in a door or a small box at the entrance to a building into which letters and small parcels are delivered. Compare post box. (mainly BRIT; in AM, usually use mailbox) N-COUNT 2. If something is displayed on a television or computer screen in letterbox format, it is displayed across the middle of the screen with dark bands at the top and bottom of the screen. ADJ