Pixels are not square.

[jumbods64]

Yeah. Although a pixel isn't a little square, sometimes it is, like in enlarged pixel art or on aliased low-res textures. And sometimes it's a fuzzy, bloomy bit like on a CRT. And sometimes it's red, green, and blue elements in that order, like in 1:1 scale LCD pixel art. A pixel is just a picture element: an element of what ever quantized picture format you're using.