Aspect Ratio is a geometrical term describing the ratio of the width to the height of a shape, and is written as a fraction. For example, a square has an aspect ratio of 1, as both width and height are identical. A rectangle which is twice as wide as it is high would have an aspect ratio of 2. The side profile of a car which is four times as long as it is high would be 4. Or a skyscraper might have an aspect ratio of 0.10, if it were ten times as high as it was wide at the base.
The term can be encountered in everyday use as a description of the 'shape' of TVs. Traditional, or Standard Definition (SD) television images have an aspect ratio of 4:3, whereas more modern High Definition (HD) Widescreen images usually have a ratio of 16:9.