The smallest addressable point of a shown image. Everything visible on a screen is made of these and none of them is visible alone.
A 1960s coinage blending pix (an informal plural of picture, from Latin pictura, a painting) with element (from Latin elementum, a first principle); the smallest addressable point of a shown image, invisible alone.
The grain every screen in the house is actually made of, too small on its own to carry a picture. What appears whole on a display is a crowd of these standing close enough together that the eye reads them as one surface.