For images such as photographs, this is the ratio between the width and height of the image. For example a square image has an aspect artion of 1:1 whereas a traditional photograph is 4:3, that is a 1/3 wider than it is high. For a digital image the aspect ratio usually refers to the ratio of the number of pixels, for example a 1024x768 has a 4:3 aspect ration, wheas a wide screen at 1920x1080 has an aspect ration of 16:9. Most commoly each pixel is square, and this is certainly the case on almost all computer screens, but occasionally in specialised applications pixels may be rectangular an have an asoect ration that is not 1:1; when this is the case the apect artio of the overall inage and the pixel rations wouydnot be the same, for example a 1000x500 image where each pixel had a 3:2 aspect ration would have an overall aspect ratio of 3:1 when printed/viewed.
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