Can someone explain Equivalent Neutral Density?

Hi,

Ive been reading the technical data sheets for the Kodak films, and for the slide films, on the spectral sensitivity curve chart, it says "densitometry: E.N.D.". Now I know the E.N.D. stands for "Equivalent Neutral Density", however I have done a very thorough google search and read books on photographic processes and no where have I been able to find and explanation of what this means. Could someone on here please explain to me what this means? I would like to know how to measure and calculate equivalent neutral density. Please get as technical as you need to and it would even be helpful to throw in some math if necessary (Im very mathematically knowledgeable so I will know what you are talking about if you throw in calculus terms).

Also, on the characteristic curve chart, the vertical axis says "density", but doesnt say how this density is measured (is it visual density? is it measuring the total amount of light that passes through the film without using any special filters? It doesnt specifiy, however I believe the density is refering to "visual equivalent neutral density". I would also like an explanation on how to intepret the characteristic curves.

I believe there is a poster by the name of Photo Engineer who will probably be able to answer all of my questions, but if anyone else can help me I would appreciate it.

Thanks, Retro Film