alt Hacker NewsA square pixel has a 1:1 aspect ratio (width is the same as the height). Any other rectangular pixel with widths different than their heights would be considered "non-square".
F.ex. in case of a "4:3 720x480" frame… a quick test: 720/4=180 and 480/3=160… 180 vs. 160… different results… which means the pixels for this frame are not square, just rectangular. Alternatively 720/480 vs. 4/3 works too, of course.
Again I think you're talking about pixel aspect ratios instead, and not physically non-square pixels, which would be display-dependent. OP only said "square pixels" but then only talked about aspect ratios, hence my confusion.