Definition - What does Aspect Ratio mean?
An aspect ratio is an attribute that describes the relationship between the width and height of an image. Aspect ratio is expressed by the symbolic notation: X:Y. The values of X and Y are not the actual width and height of the image, but describe the relationship between them.
Techopedia explains Aspect Ratio
Aspect ratio is generally used to determine the relative horizontal and vertical sizes of computer graphics. For example, if a computer graphic has an aspect ratio of 3:1, this means the width of the graphic is three times of the height of the image. Aspect ratio plays an important role in resizing. During resizing, the aspect ratio must remain same in order to keep the image undistorted. A distorted aspect ratio leads to stretching of the image.
Aspect ratio is also used to define display resolution. HDTV displays generally have an aspect ratio of 16:9.