Not definable. @Cruiser was more long winded than I am, but the definition is fine with me.
Art (and its sub-categories: music, dance, visual art, sculpture, painting, poetry, literature, etc.) is entirely subjective. One person’s art is another person’s trash.
I would only disagree with @cruiser on his last sentence. There isn’t a requirement that it be seen by anyone else. Art is the product of creative vision whether there is ever an audience or not. (Art for the sake of art)