All answers miss a very interesting point, which might be not quite what you asked, but which explains why there is such a concept as "white" though it is not a physical concept but rather a biological issue.
You will see your green plants on your table as green (nearly)regardless of the light in your room; to accomplish this, your brain has to not simply measure the spectral distribution arriving from the plant, but has to compare it with the light source - in order to "compute" what fraction of the amount of light of each frequency is reflected by the object. This is obviously a very useful feature of our neural system, since it "measures" the property of the object and not of accidental external circumstances (illumination).
How is this accomplished?
Well, you have to compare the wavelengths arriving from the object with the average incoming light in your field of view. It would be more precise to compare it with the source, but the biologically evolved mechanism has to be quick and versatile; it would need too much intelligence and delay time to search for the source (or an object that is known to be white) every time you want to check a colour.
Of course this is not done rationally, it's much simpler.
Colours are "defined" (by the brain) to be in pairs of complementary colours, which "cancel out" giving... white! White can be a mixture of red and green, or blue and yellow, etc.
When light falls on some part of your retina, the sensation of a colour is produced there - and at once the complementary colour is sent to the rest of the picture, and superpones with the real colour there! Obviously, if some frequency should be missing in the illuminating light, this is compensated by this mechanism if the objects are sufficiently randomly coloured (white on average :)). If the light is white, then all these imaginary colours cancel out, and don't have any effect.
This is why biology invented the colour white, though it does not correspond to any frequency. A very important mechanism - think about it, of how little use it would be to detect the incoming frequencies, without taking into account how they depend on the illumination. This allows you to get the sensation of a green plant even if there are no green wavelengths at all in the light present; the green is produced by the surplus of red in the light from all the other objects.
This is why the very concept of white is only useful in this context of human colour-perception. The spectra can be wildly different. Its definition is: white is what you see to be white. This may vary slightly between individuals.