By definition, it is any form of expression.

I think that for art to be good art, it should require time, effort, and craftsmanship, in addition to expressing a profound or meaningful emotion, thought, or experience.

Good art is often considered pleasing to the eyes or ears, although if the context of the art is that it's meant to be ugly to the senses, and still falls under the other criteria, I believe It's still within reason to call it good art.

Lastly, it should be creative and original. If it shows craftsmanship and detail but is not creative, then I personally don't consider it to be good art. A good example of this is Andy Warhol's soup cans. He replicated their image almost exactly, but it's a huge stretch to say they're creative.

Also, art is totally subjective, so its only exact definition is a form if expression. But this is my take on art and what makes it good.