Short form of pixel — the smallest addressable point of a shown image. A named unit rather than a concept.
Clipped from pixel, itself a 1960s blend of pix (a shortening of pictures) and element, coined to name the smallest addressable point of a digital image; px is a further clipping of that already-coined word.
Shorthand for pixel, naming the smallest addressable point in a shown image rather than a concept with a longer history. Its clipping is honest about what it is: a unit, not an idea.