To me, an artist is simply someone who earns his living through his art, or who at least produces art professionally day after day, even if he cannot live on it. In short, someone who devotes most of his time to art. Some people have found this statement very depressing; they have felt degraded, because suddenly they no longer saw themselves as artists, but felt that they were something inferior in my eyes.
The word artist is a term, however, and an artist is nothing greater or better than an art instructor or amateur painter. He is something different. A person who has turned art into his profession. Full stop, nothing more.