Light parallax is the difference between the view of object through a direct camera lens and the angle of light from a flash unit. Since the flash is separated from the lens on a fixed system, the closer the photographer is to the subject, the more parallax will occur between the flash angle and the lens axis.
Unlike optical parallax, a greater degree of light parallax is desirable.
Also see Parallax (optical).
See additional information at "Eliminate red eye."