It's quite difficult to define, actually, because I think it's used much more broadly than it used to be. I suppose what I had in mind was the traditional idea of a profession as a self-governing occupational group, that is, one which lays down its own entrance requirements, prescribes a code of 'professional conduct' with which practitioners are required to comply, and controls who is and is not allowed to practice (typically by keeping a register to which practitioners' names are added when they qualify and 'struck off' if they are naughty.) Doctors, lawyers and clergy are the classic professions in this sense; teachers aren't, I'd have thought.

The upside of 'professionalism' in this sense is that the public are protected from quacks; the downside is that the 'professionals' can exploit their privileged position to their own advantage. George Bernard Shaw called professions 'conspiracies against the laity.'