Definitions for "ND filter"
A glass filter that attenuates light evenly over the visible spectrum. It would be used to reduce the amount of light entering a lens and thus force the iris open to its maximum. This is to simulate conditions of low light in order to set the focus accurately on the desired part of the scene, for low light operation.
a gray (colorless) filter that will cut some of the light out
a simply a filter which reduces the brightness of light entering the lens (without affecting color balance - hence 'neutral')
Keywords:  story, different
a different story