A professional is a person who has completed a doctoral or law program or equivalent (a masters or bachelors degree does not infer a professional).
A professional is someone who has a professional (doctoral level) degree - a number one on the Hollingshead scale. 
A professional is a person having impressive competence in a particular activity. 
In western nations, such as the United States, the term commonly describes highly educated, mostly salaried workers, who enjoy considerable work autonomy, economic security, a comfortable salary, and are commonly engaged in creative and intellectually challenging work.   
A 'true' professional must be proficient in all criteria for the field of work they are practising professionally in. Criteria include following:
In Britain and elsewhere, professionalism is often designated by Royal Charter
In narrow usage, not all expertise is considered a profession. Although sometimes referred to as professions, such occupations as skilled construction work are more generally thought of as trades or crafts. The completion of an apprenticeship is generally associated with skilled labor or trades such as carpenter, electrician, plumber, bricklayer and other similar occupations. A related (though not always valid) distinction would be that a professional does mainly mental or administrative work, as opposed to engaging in physical work. Many companies include the word professional in their company name to signify the quality of their workmanship or service.
You do not have to have a degree to prove that you are a professional, but rather prove that you are a master of a trade set/skill, or act in what is looked to be a professional manner.
See main article: Professional sport.
In sports, a professional is someone who participates for money. The opposite is amateur, meaning a person who does not play for money, but in an academic (e.g. college football) or other private setting. The term "professional" is commonly used incorrectly when referring to sports, as the distinction simply refers to how the athlete is funded, and not necessarily competitions or achievements.
Sometimes the professional status of an activity is controversial; for example, there is debate as to whether professionals should be allowed to compete in the Olympic Games. The motivation for money (either in rewards, salaries or advertising revenue) is sometimes seen as a corrupting influence, tainting a sport.
It has been suggested that the crude, all or nothing categories, of professional or amateur should be reconsidered. A historical shift is occurring with the rise of Pro-Ams, a new category of people that are pursuing amateur activities to professional standards.