Like Alan Watts said of trying to define the personality, it is like trying to bite your nose.
There is no one definition of art. There have been many theories of art since the beginning of recorded history.
Many of these theories have been very opposed to each other. The theory is usually an extention of religious, social, etc, beliefs. For example, the idea of "art for art's sake" is morally reprehensible to moralists ... some think it is a manifesting of God in form ... seeing meaning through unity ... bringing about the sublime ... seeing only that aspect of nature that reflects blah ... representing "reality" as it is or as it should be ... that the art itself contains the vision or that it is symbolic of visionary realities ... others reject art as worshiping idols ... etc, etc ... that art reflects nature (which to some is what we see, to others what we don't see) ... it's so endless a person could go on for hours ...
"Art is a result of human creativity which has some perceived quality beyond its usefulness, usually on the basis of aesthetic value or emotional impact."
- not sure who said this quote,sorry.
"The dual nature of artworks as autonomous structures and social phenomena results in oscillating criteria: Autonomous works provoke the verdict of social indifference and ultimately of being criminally reactionary; conversely, works that make socially univocal discursive judgments thereby negate art as well as themselves."
-Theodor Adorno, from Aesthetic Theory