But there is an in=between group, surely. A professional is one who works at it full time, practising when not gigging, and earns enough to rely on this for sole means of sustenance; and some will make better livings than others, on the criteria expressed by john above.

Then there are those who attend FCs, singarounds, open mics &c. performing often to a high standard but receiving no money.

But surely there is another, in-between group. Many years ago, in 70s-80sx when I had two jobs, Head of Upper School by day, theatre, book and folk-music critic the rest of the time, getting paid for my reviews and features by The Guardian, The Times, TES, Folk Review &c. ~ I also ran an entry in the Folk Directtory, & every few weeks a club [or even occasional festival] would invite me to do a paid gig, paid at about the mid-point of what singers earned at that time. I am retired from most things, apart from an online Shakespeare journal for whom I still do theatre reviews for love not money, so not that much in touch with the scene. But surely there are still some of this last category of semi- [or even, as one might say, ¼-] professionals around?

~Michael~