Well, I am not a lawyer, nor do I play a lawyer on stage. But I do know just a small bit about what "professional" means in a strict sense in South Carolina and North Carolina, places that I have a residence.
In those states and most states in the US, no advice about other countries, the word professional has nothing to do with income or sales or peer recognition or much of anything we normally associate with the word in vernacular speech.
The word "professional" means that you have a license from the state to perform certain acts, and that you meet the state specified minimum requirements for that license, and you can be held accountable for violations of the license terms or violations of the rules, regulations, and laws regarding that profession.
Specifically, lawyers, nurses, doctors, dentists, barbers, cosmetologists, and a whole list of other "professions" have a license hanging on the wall somewhere (or a professional driver has it in his wallet most likely) that can be revoked.
Income is about money, and the IRS couldn't care less if you steal it as long as you declare it.
I work as an engineer, but I am not a Licensed PE. So there are things that I cannot do. Regardless of the level of "professionalism" I may display in my work, I am not able to claim the mantle of Professional Engineer.
Photographers are not licensed, at least not any place I live, so in South Carolina and North Carolina there are no "professional" photographers in the strictly legal sense.
For a vernacular definition, I'll offer one that my father always gave me growing up. The difference between an amateur and a professional is that an amateur may do a good job when he wants to, but a professional must do a good job when he doesn't want to.
MB