Trying to define Art is a challenging and hazardous occupation, given the elusive and slippery nature of such concept. Playing with definitions, however, can be a fun exercise to grapple with the enormous variety of artifacts, practices, and experiences called Art.
Art is not a mirror to hold up to society, but a hammer with which to shape it. —Bertolt Brecht
Art is man's constant effort to create for himself a different order of reality from that which is given to him. —Chinua Achebe
Art is sustenance. —Sarah Sze
Even the act of peeling a potato can be a work of art if it is a conscious act. —Joseph Beuys
Art allows you to imbue the truth with a sort of magic, so it can infiltrate the psyches of more people, including those who don't believe the same things as you. —Wangechi Mutu
My new favorite is the one penned by Ambrose Bierce, with his peculiarly flippant style:
Art, n. This word has no definition.