I voted for 'does art for a living' but I believe that gaining the necessary qualifications is also what defines a processional artist especially if one is considering a career in the film and games industry. I feel professional is a title given to those who are masters in the field they are in or have a paying job.
I know, personally I wish they would submit evidence that they have achieved such a status like a certificate or a photo of them in their black robe and square hat, that's what I would do. Though people never cease being students because we are learning all the time, even the masters.
I think Milne went to a special school for comic artists but don't count me on that. He's awesome no doubt!! *o* He also earns his money by working for IDW under contract so yeah I guess he counts as professional.
I'm annoyed by those self proclaimed professionals here on deviantart. Yes, we all do commissions from time to time, yes we all can draw but what enables you to call yourself rightfully a professional artist?
Man I'm overthinking this again. It's just rubbing me the wrong way when they parade around all high and mighty and most can't even answer a simple question about colors and design. :/