The Collaborative International Dictionary of English v.0.48:

Aspect ratio \Aspect ratio\ (A["e]ronautics)
   The ratio of the long to the short side of an a["e]roplane,
   a["e]rocurve, or wing.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

aspect ratio
    n 1: the ratio of the width to the height of a tv picture

The Free On-line Dictionary of Computing (30 December 2018):

aspect ratio

    The ratio of width to height of a pixel, image,
   or display screen.  Square pixels (1:1) are considered
   preferable but displays are usually about 5:4.

   (1994-11-30)