Bruce linked about that this morning. I am more inclined to ask "What is music?" than "What is art?"
Impossible questions are usually the wrong questions. "What is art?" is the wrong question. Is this art?
That's an H&E- stained microscopic slide of the human Pituitary gland. A bit of the Anterior and the Posterior, with the margin. Absolutely beautiful. You could hang it on a wall, art or not. I call it "God's art."
Another pic of God's Art below the fold, which Gwynnie stumbled on while fishing his stream the other day -