Aspect ratio refers to the proportional relationship between a video or image's width and its height. It is expressed as a fraction, where the first number represents the width and the second number represents the height. For example, an aspect ratio of 16:9 means that for every 16 units of width, there are 9 units of height. The aspect ratio determines how the image or video will be displayed on different screens or devices, with common ratios including 4:3 (standard definition), 16:9 (high-definition widescreen) and 2.35:1 (cinematic format). Changing the aspect ratio can alter the appearance of an image or video, affecting its composition and proportions.