Toothwalker;
With many raw-converters (and also built-in jpg engines in the camera) you get an "adaptive blackpoint" setting. With Nikons you unfortunately get it for "free" whether you want it or not, since the firmware scales the blackpoint to clipping (within a certain limit of course).
This can in many cases give the unfortunate (and very counter-intuitive!) effect that a lens that has LOWER global contrast is perceived as having HIGHER global contrast - since the midtones is what leaves the biggest visual impression in an image. If a lens has mroe veiling flare, the camera/converters drags that blackpoint down to "zero" - and leaves you with an over-inflated midtone contrast.
Alundeb:
Veiling flare is indeed the "wide-area" diffuse/random scatter in a lens. I think it's a good denomer, since it says exactly what is is: a "veil" of light that covers most of the image frame.
In PS you can mimic this by adding a layer of very dark gray (35 in 0-255 gamma2.2?), add mild noise to that layer, and set opacity to "screen". On top of this you put an adjustment layer, either levels or curves, and drag the BP back down. The (re)distribution of noise and contrast in the shadows is actually pretty interesting...
Even more interesting: make the gray layer tinted. Fill the gray layer with 40 blue, 20 green and 30 red, and see what happens when you pull the blackpoints down again. You get that trademark "old Sigma color".
Here you have the "colour" of a lens - it has NOTHING to do with the colour you see when looking in through the lens. If you take f.ex the yellowish colour Sigma was (is still, wrongly) known for, then you can actually model their AR layers and get the "sigma colour" of old (I don't see it in the newer lenses) from almost any good lens... If you WB for the bright parts of the image, and pull the blackpoints of each channel down to black again, you get yellowish lower midtones.
The clarity of Zeiss comes from a very wide-band AR coating, and careful attention to scatter performance. They have less blue scatter and veil than almost anything out there.