Originally Posted by Thrasher 
I think Mr Smiley is on the right track. To me the definition of art is even more basic though. It doesn't matter whether someone says something is or isn't art. As long as anyone perceives (sees/hears/feels/etc) art in a manmade thing, then it is art to that person, at least. If he communicates his perception others, then that object can becomes art to those he talks with.
Hence the state of the art market, where value pretty much boils down to supply and demand.