A term that is used to describe the blank bars (usually black in colour) all the way around an image when the letterbox effect and pillarbox effect occur simultaneously. It is most often the unintended result of viewing non-anamorphic letterboxed material on a widescreen display or when a 16:9 film is set to 4:3 (letterbox), but then shown on a 16:9 TV or other output device."
Anamorphic vs. Non-anamorphic (1.85:1)
Anamorphic vs. Non-anamorphic (2.35:1)
"....or when a 16:9 film is set to 4:3 (letterbox), but then shown on a 16:9 TV or other output device."
The reason a 16:9 film (or other program) is converted to letterbox 4:3 is to accomodate people with older 4:3 TVs. When they see it, it is simply letterboxed. But then when you see it on your 16:9 TV, the TV automatically pillarboxes the image, resulting in black bars all around the picture. (Unless your TV is set for grey instead of black, in which case you see black on top and bottom and grey on the sides.)