Found this somewhere - it's the actual definition of "professional" given by the National Labor Relations Act, (Title 29, Chapter 7, Subchapter II). I've paraphrased it below:
"Professional employee" means "(a) any employee engaged in work (i) predominantly intellectual and varied in character as opposed to routine, mental, manual, mechanical, or physical work; (ii) involving the consistent exercise of discretion and judgment in its performance; (iii) of such a character that the output produced or the result accomplished cannot be standardized in relation to a given period of time; (iv) requiring knowledge of an advanced type in a field of science or learning customarily acquired in an institution of higher learning or a hospital; or (b) anyone who is training to become a professional."
I think this gives everyone some food for thought. Being a professional does not mean that you do something for money. In fact, you can be a professional and work for free - a problem that's plaguing the photography community today.
What's important in being considered a professional (by this standard, which is used to define exempt vs. non-exempt employment) is not that compensation is earned, but that the work is varied enough in its output that it goes beyond simple manual labor.
Using that definition I propose that a "professional" photographer is someone who doesn't just photograph the things in front of them (a pretty girl, a lovely forest, or a far-off location), but someone who creates varied and diverse imagery through the use of photographic technique.