The ratio of the dimensions of a screen resolution. Always listed as the longer dimension first.

The most common aspect ratios that games will deal with are 4:3 (standard definition television and computer monitors) and 16:9 (wide-screen television and computer monitors).

This tag specifically refers to screen aspect ratio, not for aspect ratio used for in-game objects. While questions deal more often with scale, differences in aspect ratio actually figure into completely different screen real-estate, which has a much different set of problems.