The width-to-height ratio of a rectangle.

For example:

If a rectangle is 640 by 480 the aspect ratio is 4:3, or (1.33333:1)

If the rectangle was 300 by 300 (i.e. a square) the aspect ratio would be 1:1

In Creator the Aspect Ratio is often used to ensure that images or videos are displayed in the correct ratio irrespective of the size; so if the object is scaled the image/video inside that object's bounding box will still be displayed with the correct aspect ratio. If the Stretch to Fit option is used instead the image/video will always fill its bounding rectangle, irrespective of the effect on the image aspect ratio.