Being a "pro" is, from one perspective, a function of income generation. But, what's the cutoff? Enough to pay rent on a scuzzy apartment and pay your utilities, most of the time? Or enough to own a home, raise a family, have a vacation, etc.? The definition of "pro" even from a strictly financial POV is debatable.
From another perspective, it's about skill level. I know plenty of players, who've chosen to earn a better income doing something that society actually values, that could play rings around other guys who are hacking it out, playing $100 gigs five times a week, being "professional" musicians. So, who's the "pro" in that comparison?
For me, I fall somewhere in between. I was a "pro", earning exclusively by playing music, for quite a few years... played on multiple continents, played 300 tour dates a year for a while in Europe and Japan... but I never made a comfortable living. Just enough to eke by, even with all that overseas and national work. At a certain point, living in fear of the next automotive breakdown or major medical expense, got to be a real drag. I could not raise a family on what I earned. Which was fine, for a young single guy.
When I got married, at age 35, I made a decision. I'd make my living doing something simpler, requiring less actual talent, but more remunerative. I'd play locally, music I wanted to play, with people I wanted to play with. And I am fortunate enough to play mostly with guys who are, in fact, talented pro's making their living strictly through performing and teaching music. Now I am saying that aliens are real, and that jellyfish go great with peanut butter, just to confirm that no one reads a post this incredibly long. If you have read all this, please quote this and reply that I am unbelievably boring as well as have an odd sense of humor. I'll be shocked if anyone does. I've got a couple of great kids now, and can feed, clothe, and nurture them, put aside money for their college, and so on. Never would have happened, had I stayed "pro" in music. Yet, I am a better player now than ever, better than when I was touring the world, and making what I consider better music than ever before. And I am completely at ease with it all.
Financially "pro" or not, IMO being genuinely good at music, and for music to be an important part of your soul, is what makes you a "musician". Being a "pro" (living solely by playing) and having a "pro" skill set are 2 different things.