I still like that definition, but I have an interesting thought to add, which leads to some useful Art Historical insights. The thought is this: since the beginning of Modernism, in the mid-nineteenth century, Art of the Western Tradition has, on some level, been about challenging the definition of Art.
This doesn't mean that every single artwork made in Europe and America since 1850 has tried to challenge the definition of art, just almost all the ones that have become famous.
An artist can challenge what is "acceptable" as subject matter, as medium, as source material.
Challenge #1: Art doesn't just have to be about important people, but can be about the everyday, humdrum workers of today's world.
Gustav Courbet, The Stone Breakers, 1849, oil on canvas, (original destroyed 1945)
Challenge #2: A painting doesn't require many meticulous layers, but can be made outdoors, in one layer, in a single sitting, and still be art.
Claude Monet, La Grenouillère, oil on canvas, 1869, Metropolitan Museum of Art
Challenge #3: Paint can be applied to a canvas not just to try to create illusionistic space but also to emphasize the flatness of the canvas and the reality of brushstrokes.
Paul Cezanne, Mont Saint-Victoire, oil on canvas, 1902-1904, Philadelphia Museum of Art
to be continued...