The ratio of width to height of an object. Stating the relationship of one side to the other, it is widely used to describe the shape of a TV or computer screen. For example, the aspect ratio of a standard screen is 4:3, which is a relatively square rectangle. The 4:3 means "4 to 3," or four units wide to three units high. High-definition TV (HDTV) has a 16:9 ratio, which is a wide rectangle closer in width, but still not as wide, as most cinema screens.

Another way of expressing the 4:3 and 16:9 ratios is 1.33:1 and 1.78:1, but these latter designations are used mostly for cinema formats, not TV (see below).

Standard or Wide Screen?
The common measurement of a computer monitor or TV is the screen's diagonal measurement in inches. However, a 20" screen does not disclose whether its layout is 4:3 (square) or 16:9 (wide). See letterbox, resolution, HDTV display modes and anamorphic DVD.