Ok, let's see if I can come up with something coherent. . .
Art is a hard thing to nail down with one definition because its purpose has changed over the ages. It has transformed from primarily descriptive to interpretive. Cave paintings tell a story of a great kill (
http://www.minervatech.u-net.com/illos/lasc2.jpg). Greek temples depict (architecturally) the traits of the god or goddess (
http://www.ludvigsen.hiof.no/webdoc/inet93/cyberspace.ill/parthenon.gif). Stained glass windows and the ceiling of the Sistine Chapel (
http://sun.science.wayne.edu/~mcogan/Humanities/Sistine/Ceiling/Ceiling.2.jpg) tell Biblical tales. The Mona Lisa (which really is in no way a phenomenal painting) is simply a portrait of a client (
http://www.ai.mit.edu/people/ltk/Photos/mona-lisa.jpg).
Along the way painters started to put symbols into their works or alter proportions to show hierarchy. 'Round about the 19th century they began to experiment with new techniques and really look at the relationships of light and colors (
http://library.thinkquest.org/27356/media/paintings/caravaggio.conversionofsaintpaul.jpg). Impressionism gives the painter's simple impressions of a subject. It's not intended to look real (
http://www.economia.uniroma2.it/conferenze/icabr/img/monet.jpg). Those abstract impressions developed into an art more focused on the study of color theory. Mondrian's primary colored squares are more graphic design compositions than pure art, but they have the purpose of causing people to look at color and composition differently (
http://titan.glo.be/~gd30144/mondrian.jpg). Most modern art is intended to interpret something and change the viewers perpective of it. It's like the students in Dead Poets Society standing up on their desk to look at the room differently (
http://www10.pair.com/crazydv/weir/dps/pics/assorted1.jpg). Duchamp's Fountain (which is a urinal) is supposed to make you think about what really makes a fountain (
http://www.thespoon.com/art/about/images/duchamp-fountain.jpg). Whether this is a useful paradigm shift is is up to you.
One of the most universal definitions of art is something that responds to your emotions. Michelangelo's Pieta (Mary sitting with dead Jesus draped over her
http://www.christusrex.org/www1/citta/0-Pieta.jpg) surely responds to a Christian's emotions. The blended rectangles of Rothko (
http://www.poster.net/rothko-marc/rothko-marc-orange-and-yellow-2102369.jpg) can also evoke emotions though. Colors have been shown to have a psychological effect on people, Rothko and others are exploring that. Many contemporary works are psychological. Putting fish in a blender is a way of looking at the morals of humanity.
So I don't have a definition for you, but I think one of the major points of modern art is to remain open minded. And remember, most progressive artists were hated by their contemporaries. Michelangelo was criticized for making his sculptures look too scarily real. Monet's works were shunned as trivial.
