Dr. Bill and Geoff have both stated correct meanings for the term. Not surprisingly, there is more than one meaning. There is the old, narrow meaning which Geoff alludes to, but there is the meaning of someone who does for money, or for a living, what others do for amusement or pro bono publico. There are professional gamblers and there are those who play penny-ante poker every 3rd Wednesday evening. And whereas it was originally an honorable term, it can now take on a distinctly nasty connotation, as in the expression "professional do-gooder".