Doing it for a
recognizable part of your income, notwithstanding whatever unprofessional things you might be doing in that trade or your awesome work ethic.
No money... Not a pro, regardless of your talent set.
And making money on the side while pursuing some other career doesn't count, either.
It's like this: you might be getting money for being the best college first baseman that ever lived, but you ain't "Pro". You might advance to the Minor Leagues, and now you might be considered a "pro" because you are certainly doing it for a living. Everybody knows that you are a "Pro" baseball player when you make it to the big league, regardless of the team you might happen to be playing on. What position you might be playing will also tell everyone your importance to the team, too.