I don't think art as a whole can be defined. There is no single something that makes art art, it simply is. It's more of an experience, an internal feeling that you know you're seeing something that is beyond simple definition. Sure you can categorize art, explain the processes one uses to create each type of art, but I feel that the essence of what art itself is goes beyond simple definition.
For example, one can hear a symphony played by the greatest orchestra in the world and exclaim it was a wonderful performance an be moved to tears. Now a musicologist could explain how the use of a cresendo, the perfect blend of harmonies and orchestration creates a sound that is musically perfect, but it's impossible to explain why that cresendo, that chord, moves us in the way it does. It touches something deep in side that we can't define.
I hope that made some sense at all.