I googled this once and went deep down a rabbit hole. Consolidated my favorite discoveries here to save you time. I added a work from each artist that I think best exemplifies their definition of art.
"Art is a lie that makes us realize truth" - Pablo Picasso, Spanish Artist (1881-1973)
"Art is not what you see but what you make others see" - Edgar Degas, French Artist (1834-1917)
"Art is a line around your thoughts" - Gustav Klimt, Austrian Artist (1862-1918)
"There is only one valuable thing in art: the thing you cannot explain" - Georges Braques, French Artist (1882-1963)
"Art is what you can get away with" - Andy Warhol, American Artist (1928-1989). Warhol actually took ownership of this statement from Marshall McLuhan. (Source)
"Art is the Queen of all sciences communicating knowledge to all the generations of the world” - Leonardo da Vinci, Italian Artist & Scientist (1452-1519)
"Art is a harmony parallel with nature” - Paul Cezanne, French Artist (1839-1906)
"Art is an effort to make you walk a half an inch above ground.” Yoko Ono, Japanese Multimedia Artist (b. 1933)
‘Art is a human activity, consisting in this, that one person consciously, by certain external signs, conveys to others feelings he has experienced, and other people are affected by these feelings and live them over in themselves’ - Leo Tolstoy, Russian Author (1828-1910)
"Art is the unceasing effort to compete with the beauty of flowers – and never succeeding" - Marc Chagall, Russian-French Painter (1887–1985)
"Art is either a plagiarist or a revolutionary" - Paul Gauguin, Peruvian-French Artist, (1848–1903)
"Filling a space in a beautiful way. That's what art means to me" - Georgia O'Keeffe, American Painter (1887–1986)
"What I dream of is an art of balance, of purity and serenity, devoid of troubling or depressing subject matter, an art which could be for every mental worker, for the businessman as well as the man of letters, for example, a soothing, calming influence on the mind, something like a good armchair which provides relaxation from physical fatigue" - Henri Matisse, French Artist (1869-1954)
"I don’t think art is elite or mysterious. I don’t think anybody can separate art from politics. The intention to separate art from politics is itself a very political intention" - Ai Weiwei, Chinese Artist (b.1957)
"What is art? Art grows out of grief and joy, but mainly grief. It is born of people’s lives" - Edvard Munch, Norwegian Artist (1863–1944)
"Art has to move you and design does not, unless it's a good design for a bus" - David Hockney, British Artist (b. 1937)
