I think of art as a form of communication. While art's lack of a codified set of meanings means it can rarely produce as specific a meaning as a codified form of communication such as spoken language, I find that it can also pack a much greater emotional punch than simple speech. That's not to discount art that involves speech in any way, as speech can be used to create art in the same way that paint or metal can. I also think art is defined by the user, if someone identifies something as art they have made it into art themselves as people will then begin attempting to assign meaning to it.