Aspect Ratio
The proportional relationship between an image’s width and height (e.g. 16:9).
Aspect ratio is width divided by height. Common ratios are 1:1 (square), 4:3, 3:2 and 16:9 (widescreen). Keeping it constant while resizing prevents stretching.
Lock or unlock it in Resize by Pixel, or crop to a ratio with Crop Image.