This is such a big question. I don't know if I have an answer, but I have a ****load of words to throw at it. Sometimes you can only elude to what something is, point to what it is not, and then infer what's actually there.
I'll start with philosophy.
This is going to be a total crash course in Platonic thought. Read The Republic if you want the deep explanation.
First, Plato was all about the division between the visible realm from the intelligible realm. The intelligible realm consists of "the good," which sort of illuminates eternal forms such as Beauty, Truth, Justice, etc. The visibile realm, however, consists of all sensations we are able to detect. The intelligible realm can only be "sensed" through dialectic and abstract thinking, while the visible realm can be touched, tasted, felt, etc. Objects existing in the visible realm (including all art) are only pale shadows of what exists in the intelligible realm. Since our senses are faulty and may give us false impressions, truth can only be found in the intelligible realm, which (for Plato) is at all times preferable in pretty much every way imaginable to the visible realm.
Now, Plato didn't say all art is imitation. He said representational art is imitation, and thus deception. Since objects in the visible realm are already untrustworthy and faulty in comparison to what they are shadows of in the intelligible realm, representations of visible objects are even worse. Representational art is shadows of shadows, and Plato felt this misled people even further from the truth, which could only be found through mathematical study and dialectic. For Plato, though art may try to represent ideas of beauty, it will never be beauty itself. Since both music and visual art can be considered "beautiful," we see that "beauty" is located neither in vision nor hearing, but in some intangible quality they can at times represent.
Abstract art, for Plato, is less faulty since it more closely aligns itself with eternal forms, rather than objects in the visible realm. Although there wasn't much abstract visual art in ancient Greece, they did have instrumental music, which Plato considered to be abstract. Lyrics he handles differently, as they fall into the category of poetry and he's pretty hesitant to criticize that whatsoever since it was held pretty high in his day (on par with the Bible depending on the poet, since poets were thought to be divinely inspired). It would be interesting to see what Plato would think of the abstract expressionists or other highly non-representational visual arts found today, since they were nonexistent in his own time. Even if you try to represent a circle, you are being deceptive from his perspective. You can't create a circle, no matter how perfect or what tools you use. No perfect circle can exist in the visible realm. The only true circle that exists for Plato is in the intelligible realm, and its most accuracte representation is stated, "x² + y² = r²"
So much for Plato. Other philosophers vary, and I'm not entirely sure they're the ones to look to as experts in this matter, if there even are experts. Schopenhauer argues that the only human state in existence is suffering, and what we confuse for happiness is just temporary relief from suffering. Life is suffering, and it is ultimately inescapable except through enlightenment (yes he rips off Buddhism/Hinduism in many respects). However, art temporarily saves us from suffering. So if you're a Schopenhauerian, you could define it as, "that which
temporarily saves us from suffering."
You could define it as a visual record, though the camera and later video have made that notion somewhat obsolete. You could claim it's a social or emotional record, with inherent depth and meaning that photography couldn't touch, but then you have to keep in mind the subjectivity of selecting what picture to take; photographs are not objective. You can say that art is emotional expression, but then every human act and the results of them would be considered art and the definition would lose meaning. Where do you draw the line?
In my opinion, though probably no one will have read this far or care if they did, is that the definition of art is ultimately subjective. I can't get any closer than that, and I'll be getting my BFA in drawing/painting some time next year. For me, it's alternately life, an obsession, a deathmatch, experience, and existence itself.