Frankly, there is no such thing, objectively speaking, as "art".
"Art" is such a wholly subjective notion that you can get a hundred people together, ask them what they think "art" IS, and within seconds of talking about it you'd have a hundred completely different and mutually exclusive answers. In science, when something is THAT impossible to quantify, it's typically called "pseudoscience" or "bullshit".
Using the most basic, definition of "art" (your Mona Lisas, your Michelangelo's David, et all) objectively one could argue that "art" is another word for "something we find aesthetically pleasing or interesting to look at". Or in other words, something pretty.
But if you put a gun to my head and made me answer what is "art" in gaming, I'd say something like the Modern Warfare or Mass Effect series which manage to emulate in a very real and total way popular action movies and science fiction books, respectively, tend to play out. This is no mean feat despite what the COD-hating game-snobs will tell you (I'm looking at you Bob Chipman), since these are very big and very expansive genres. Mass Effect more than anything else captures the feel of a sci-fi novel of the likes of the Lensman series, Revelation Space or the Culture series...something which, if you'd asked me a few years ago was possible with an interactive game, I would tell you was all but impossible.
So yeah...TL;DR version, "art" can be anything but typically it revolves around personal aesthetics and what we find appealing and-or fascinating.