Look up: ND Filter

1. ND Filter
   A filter that attenuates all colours of light in the visible spectrum, equally.
   Found on http://www.zoo.co.uk/~z0001325/Glossary.
   
   
2. ND Filter
   Abbreviation for Neutral Density Filter. Attached to the front of the camera's lens, it reduces the amount of light entering the lens. When light levels are strong, such as when outdoors in sunshine, attaching an ND filter allows you to use both your maximum aperture and slower shutter speeds.
   Found on http://www.panasonic.co.uk/html/en_GB/Te
   
   
3. ND Filter
   ND (Natural Density) filter filters down to one-tenth the quantity of light to prevent the picture going out of focus under bright conditions. The digital camcorder automatically senses the need for ND filter and indicates it in the viewfinder
   Found on http://www.encyclo.co.uk/visitor-contrib
   
   
4. ND Filter
   The ND (Natural Density) filter reduces the amount of light going into the lens to prevent the picture going out of focus under bright conditions. The digital camcorder will automatically sense the need for ND filter and provide an indication in the viewfinder.
   Found on http://www.sony.co.uk/glossary/ShowGloss
   
   
5. ND Filter or Neutral Density Filter
   A filter that attenuates light evenly over the visible light spectrum. It reduces the light entering a lens, thus forcing the iris to open to its maximum.
   Found on http://www.rodsmith.org.uk/photographic%
   
   
6. ND Filter or Neutral Density Filter
   A filter that attenuates light evenly over the visible light spectrum. It reduces the light entering a lens, thus forcing the iris to open to its maximum
   Found on http://www.digitalexposure.ca/sub1.html