letterbox

Short definition from Wiktionary

Noun

1. 1
   A box into which mail is put: A collection point for mail intended for onward delivery, a secure box or receptacle for this purpose.
2. 2
   A box into which mail is put: A delivery point for mail, a box, compartment or slot for this purpose.
3. 3
   A hidden container that holds a logbook and rubber stamp, found by following clues as a form of recreation.
4. 4
   A rectangular hole or slot.

Adjective

1. 1
   Transferred to home video formats while preserving the original aspect ratio, having black bars above and below the picture area.

Verb

1. 1
   To transfer a widescreen motion picture to home video formats while preserving the original aspect ratio, with the placing of black bars above and below the picture area.
2. 2
   To hunt for letterboxes (containers with logbook and rubber stamp) by following clues.