letterbox

noun

1. Also letter box. Chiefly British. a public or private mailbox.
2. Digital Technology, Television. a technique for displaying a wide-screen film or landscape video on a narrower screen by reducing its size but retaining the aspect ratio, with black bands filling the screen above and below the picture (often used attributively):letterbox videos.Compare pan and scan, pillarbox(def 1).

verb (used with object)

3. Digital Technology, Television. to display (a film or video) by using the letterbox technique.