How to define white color scientifically? I'm sorry some of you may mark this question as duplicate, but 
even though I searched over about this I couldn't get satisfacory answer.
Also I think this question is more related to biology, but I dont think I would get a satisfactory answer on biology forum.
I have had much intesest in color perception. I learned that color is just illusion of human brain to interpret electromagnetic spectrum. But though I'm physics grad student, I never questioned myself why there is no white color in visible ray spectrum. I realized that today. Sure thing, I learned when I was young that white color is 


*

*what we see when we see an object that reflect all the light.
But what do we mean by 'all the light' here? every illuminant has different spectral power distribution so even if an object reflects them all what we see would depends on illuminant. And what does it means that it reflects every light? surely not all the electromagnetic spectrum. Only visible rays? reflects 400nm but not 390nm? I'm not sure about that.

*Equal composition of RGB, three primary colors.
If you see the CIE color space picture with sRGB gamut on it, you would question whether we can call these colors 'primary'. 
 
Yes we humans have three types of cones, but they don't correspond exactly to each color of RGB.
I thought white should be a extreme on certain measure (like lightness in 'conventional' term) but Im not sure which is corresponding scientific term. Or isnt white such a special color? just some 'appropriate' mixture of colors like RGB?
I read some articles about this on Wikipedia but I'm not sure I understood them because they use many technical terms in color science.
 A: "Whiteness" is more of a biological and perceptual quality rather than something derivable from physical principles. It means the presence of light across all the humanly-visible range, or at least within the three sub-ranges that trichromatic people can perceive. 
White light is also not necessarily three separate colours - dichromatic people perceive white when only a narrower range is involved (or two more isolated ranges, depending on the nature of their dichromacy), and tetracromatic people can differentiate four separate sub-ranges.
A: 
1) what we see when we see an object that reflect all the light. But what do we mean by 'all the light' here? every illuminant has different spectral power distrubution so even if an object reflects them all what we see would depends on illuminant. And what does it means that it reflects every light? surely not all the electromagnetic spectrum. Only visible rays? reflects 400nm but not 390nm? Im not sure about that.

"All the light" simply means that each and every colour in VIBGYOR is reflected. The percentage of reflection is not needed here. White light is a combination of all the colours in the visible spectrum. In the image you've posted, I believe you've seen the white in the exact middle. That is because the three primary colours first make the secondary colours, and all of them together make the white colour. It doesn't have to reflect all the wavelengths, because that is not possible, but there should be different wavelengths that correspond to each colour in VIBGYOR. For each colour, there is a range of wavelengths. Any wavelength in that range may be reflected.

2) Equal composition of RGB, three primary colors If you see the CIE color space picture with sRGB gamut on it, you would question whether we can call these colors 'primary'. Yes we humans have three types of cones, but they doesnt correspond exactly to each color of RGB. I thought white should be a extreme on certain measure (like lightness in 'conventional' term) but Im not sure which is corresponding scientific term. Or isnt white such a special color? just some 'appropriate' mixture of colors like RGB?

Perception of white colour means that our eyes identify each of the different colours of the visible spectrum, and when the brain gets the signal that each and every shade in VIBGYOR is present in the incident light, it takes it as white light. For example, the eyes send signals for the three primary colours RGB, and then for each of the secondary colours. With all the signals together, the brain interprets it as white. Don't ask me how the brain differentiates between the signals and understands all the shades are present, because that'll be biology and not physics.
