I really like Ellen Dissanayake's definition of art: "making special". An anthropologist who studies art's evolutionary origins, she sees art as a fundamental human behavior.

My position, lone as it might be, is that making important activities special has been basic and fundamental to human evolution and existence, and that while making special is not strictly speaking in all cases art, it is true that art is always an instance of making special. To understand art in the broadest sense, then, as a human behavioral proclivity, is to trace its origin to making special, and I will argue that making special was often inseparable from and intrinsically necessary to the control of the material conditions of subsistence that allowed humans to survive.