What is an Art lens?

Anyone know what an Art lens is? 

I have seen certain lenses classified or categorized as “Art” lenses. Sigma
actually has a collection of lenses that it categorizes that way. One example
is the Sigma 50mm DG HSM f/1.4. These so called art lenses generally are higher
priced than those not designated in this fashion, though there is the
occasional exception, i.e. Sigma has a 19mm f/2.8 that they classify as an art
lens but it sells for $199.
Do these lenses have any significant features that one wouldn’t find on a non-art
lens? Is any distortion at a minimum or are the optics especially good perhaps?

Does an art lens have functionality or features that make using it difficult?
 
Take the first example I cited above, the 50mm f/1.4. This lens retails for about
$950. So why would any photographer choose this 50mm lens over many others that
cost half or more than half as much? Sigma themselves make other 50mm lenses
that cost way less.
 
Consider the Zeiss 50mm f/1.4 Planar T lens. I have not seen Zeiss classify this lens as
an art lens but it is often compared to the Sigma 50mm I mentioned above. It
sells for around $625, quite a bit less money than the Sigma I mentioned but
still pricey for a 50mm f/1.4.
 
While I have seen the word art used to classify one lens or another in photographic
circles (e.g. forums, review articles), Sigma is the only company I know of
that uses this word in its advertising for some of its lenses. 
So is this use of the word art by Sigma simply a marketing approach for a line of
its lenses? Or does the classification or categorization of a lens as an art
lens have any real meaning?

Well, one might call that "marketing".

As for differing price points, clearly not all lenses perform comparably. There are parameters of image quality that are affected by a lens, and those parameters vary by manufacturer and lens. One must explore each offering to ascertain it's suitability, right?

Me, by way of example, I find the chromatic aberration produced by the Nikon 50mm f/1.4 AF-S G lens to be objectionable. And that's a $500 lens that is (arguably) highly admired. Net: I'd look at the Sigma ART first, should I find a need for one of those.

Lateral Chromatic aberration (not longitudinal) can be corrected in a number of different ways both in camera & in post, thus that , to me, isn't a factor as much as it might be for you... The cost is a much bigger factor in my book. I bought a Nikkor AF 50mm F1.4 off a person for $35.... Best $35 I ever spent. He had complained about the same issue & ,while I had informed him of the corrective measures he could take to rid images of it, he preferred to buy a newer iteration (G) of the lens from Nikon (which ,supposedly suffers from the same optical shortcomings). He messaged me privately about purchasing the older version at that price & I hopped on it...

Consider Sigma's "ART" series the same as other lens makers Pro Glass. All brands have consumer grade lenses and then they have their pro line lenses.

Screamin Scott wrote: I bought a Nikkor AF 50mm F1.4 off a person for $35.... Best $35 I ever spent.

 Nice!