my two cents:

Professional can be an adjective, as in a professional actor, a professional tennis player, or a professional writer. In these instances the word differentiates between the hobbyist and the person who earns a living from the activity. One interesting case is professional soldier, which implies a level of training and career intention beyond a typical draftee or enlistee, even though they too are paid for their work.

As a noun the word gets used pretty loosely. For example, in personal ads you'll often see something like "professional white female seeks professional, non-smoking white male, age 45 to 60...." One gets the impression that they mean "upper middle class," and bakers, carpenters, steam-pipe fitters, welders, stevedores and other tradesmen need not apply.