Pillarbox

Usually seen in games intended for 4:3 aspect ratio after widescreen became more common.

this is generally a solution where the level/area designs are deemed to give too much information to the player by showing too much that was not intended to be available. For example a puzzle or maze might become trivial with the added vision on 16:9 display when playing a game intended for 4:3 display. Or you might see some things ahead of time that you shouldn't be aware of, such as an ambush or such.

See also: letterbox