Definitions differ. Some people define a professional photographer as someone that makes money from taking pictures.
Others consider that a professional photographer should have photography as his main source of income.
What we can say for sure is that a pro is not someone encumbered with pro gear.
A pro has a professional attitude, the knowledge necessary to do a job, and the humility to realize when the job is outside his area of expertise.
In short, a pro does not let a client down.
Lastly, a pro needs to be aware of the legal aspects of his activity: insurance, model release, bizarre local laws.