letterboxed
adjective
let·ter·boxed ˈle-tər-ˌbäkst 
variants or less commonly letterbox
ˈle-tər-ˌbäks
of a video recording
: formatted so as to display the full rectangular frame of a wide-screen motion picture
letterboxing
ˈle-tər-ˌbäk-siŋ
noun 
Word History
Etymology
perhaps from the resemblance of the picture on the TV screen or the bands above and below the picture to slots in a mailbox
First Known Use
1982, in the meaning defined above