Whence it comes to pass that the high and formal discussions of learned men end oftentimes in disputes about words and names; with which (according to the use and wisdom of the mathematicians) it would be more prudent to begin, and so by means of definitions reduce them to order.--From The New Organum--
Many art classes have a huge discussion regarding the question "What is Art?" And many buildings are full of things which are supposed to make us question what art is. It's perfectly clear where this line of thinking is going to end--with lots of stuff people aren't sure if they can throw away. So I'll take a page from Francis Bacon and make up a definition, which probably isn't even original.
Art: Something made by humans for the purpose of expression.
Whenever someone asks,"What is art?" I'll immediately shoot out that answer.
I can even use the definition to determine how good a piece of art is (How expressive is it, and do I like what is trying to be expressed?).