means The smallest individually controllable point of light or color in a digital image or display screen, the tiny building block from which every picture is assembled.

from A 1960s coinage from the world of digital imaging, blending "pix" (a slangy plural of "pictures," itself film-industry shorthand) with "el" clipped from "element" — so a pixel is literally a "picture element." The compact word spread alongside the technology, and it later spawned a sibling, "voxel," for the same idea in three dimensions.