This definition has nothing to do with skill, accomplishment, goals, ability, or anything else other than a state of mind and the resulting natural flow from that state of mind.
An amateur faces challenges (real or imagined) in their quest to achieve X and they become derailed, unfocused, and diverted. A professional may experience the same exact challenges but, due to their state of mind, continue on diligently working away to achieve X regardless of outer or inner reasons to stop.
An amateur in any field procrastinates, finds excuses, creates alternative work... anything to keep from doing and achieving what they deep down feel drawn to do.
A pro just does the work. The ugly work, the dirty work, the unmarketable work... they keep honing and developing what they are drawn to do regardless of if anyone else in the world feels their work is valuable or important.
As someone who has 1001 interests I have to ask myself - where am I just doing busy work and where am I really drawn?
Do I have so many interests simply because I'm very successful at running away from becoming a pro at the areas I'm really passionate about? I'm making myself too busy doing B-Z that I can only dabble with A - even though A is, if I'm honest, my most passionate area?
Do I really want to speak multiple languages (or run an adventure race, or compete in Mt. Trail, or write a book, or learn pottery, or create art, or or or)? I certainly do not have the habits a pro would utilize to do any of these things.
Am I hiding from the professional effort needed to accomplish these things OR am I wasting time playing with this and that on a surface level to avoid digging down into the area(s) I should be truly focused upon?
The onion layers are many.