I always refer back -- via Jacques Maritain's Art and Scholasticism -- to the scholastic definition of art as "right reasoning about a thing to be made." Note that the noun in the definition is "reasoning," not "thing." Art happens, if it happens at all, inside the artist's head -- or, more generally, between his head and his hands.
The "thing to be made" is, under this definition, not "art" itself, but what art produces. It is a "work of art" in the same sense that a school essay written the night before it is due is a "work of desperation." To say of a painting, "This is art," is to speak analogously of something internal to the artist signified by the painting. It's like saying of a fire extinguisher in a kitchen, "This is prudence."
Nobody is obliged to accept that definition of art, but I would submit that it is more useful than, "I know it when I see it." Notice, for example, how it untangles that thorny Twentieth Century question, "But is it art?"
By "untangles," I mean "replaces." We can say:
"But is it art?" is not a well-formed question. A well-formed question is, "What is the reasoning of the artist signified by this artifact?" From there, we can ask two further questions: "Is this a well-made signifier of the artist's reasoning?" and "Does this signify something we value?"
When you consider all of these as distinct through related questions, you've got something to talk about.