The ratio between the width and height of the TV picture on the screen. There are two common TV screen shapes that most people will recognise - for television monitors it is either 4:3 ('standard') or 16:9 ('widescreen'). NTSC analog TV systems use a 4:3 aspect ratio, while ATSC uses the wider 16:9 aspect ratio.
While most new HD programming is in 16:9, a significant amount of TV broadcasts are still sent in the conventional 4:3 ratio. When watching 4:3 programs on a HDTV 'pillar boxing' may occur, with black columns displayed on the left and right of the image.