Most digital imaging systems display an image as a grid of tiny, square pixels. However, some imaging systems, especially those that must be compatible with standard-definition television motion pictures, display an image as a grid of rectangular pixels, in which the pixel width and height are different.
The physical pixels of a display device are most commonly on a grid that is equally spaced both horizontally and vertically (which is what square pixels most commonly means in a display/imaging context, but the pixels themselves are not required to be of any particular shape.