ND is the universally used abbreviation for neutral density, referring to a category of optical filter or other light reducing medium that reduces the intensity of light passing through it uniformly across the visible spectrum without selectively absorbing or altering any particular wavelength or colour of light. The neutral in neutral density refers to this spectrally unselective nature of the light reduction - an ideal neutral density filter transmits all wavelengths of visible light equally, reducing overall brightness without introducing any colour cast or shift in the colour balance of the image.