The aspect ratio of an image is its width divided by its height (usually expressed as "x:y"). For instance, the aspect ratio of a traditional television screen is 4:3, or 1.33:1. The aspect ratio of a standard 35mm film frame is around 1.35:1, although cameramen sometimes use only the part of the frame which will be visible on a television screen. Letterboxed videos are frequently in 16:9 aspect ratio. (Or about 1.78:1)
Within the motion picture industry, the convention is to assign a value of 1 to the image height, so that, for example, a Cinemascope frame is described as 2.35:1 or just "2.35". This way of speaking comes about because the width of a film image is restricted by the presence of sprocket holes and, usually, an optical sound track on the projection print. Development of various camera systems therefore centers on the placement of the frame in relation to these lateral constraints; the height of image can be adjusted freely, so the ingenuity goes into getting different widths. One clever widescreen process, VistaVision, used standard 35mm film running sideways through camera gate, so that the sprocket holes were above and below frame and the width was not restricted. The most common projection ratios in American theaters are 1.85 and 2.35.
The term is also used in the context of computer graphics to describe the shape of an individual pixel in a digitized image. Most digital imaging systems use square pixels--that is, they sample an image at the same resolution horizontally and vertically. But there are some devices that do not, so a digital image scanned at twice the horizontal resolution to its vertical resolution might be described as being sampled at a 2:1 aspect ratio, regardless of the size or shape of the image as a whole.