The relation of width and height, as considered from a specific direction. An aspect ratio may be represented in a colon format X:Y, such as 3:2. The numbers do not necessarily refer to actual width and height but, rather, the relation between the two.
An image with an aspect ratio of 3:2 might have a resolution of 900×600 or 1200×800 or another size, each being a scale of the ratio.
In ProPhoto, aspect ratio is identified as width:height so that 2:3 is a different orientation than 3:2 – the former being a ‘portrait’ orientation, and the latter being a ‘landscape’ orientation.