Professional
A professional does something as a profession, or receives payment for some activity. The adjective "professional" can indicate that someone has great skill in a craft or activity, or that something demonstrates such skill.
To conduct oneself as a professional (exhibiting "professional behavior") would indicate that the person's actions remain in accordance with specific rules, written or unwritten, pertaining to the standards of a profession.
The opposite of "professional" is "amateur" (disparagingly: "rank amateur"). In many cases someone can perform the exact same craft or task, and the only difference between a professional and an amateur consists in the payment of the one but not the other. Athletes in some sports furnish examples of this.
In many areas a person must overcome a barrier before gaining recognition as a professional.
Such barriers include academic degrees, certifications, licenses or completion of an apprenticeship.
Professions with such barriers include those of doctors, engineers, lawyers, plumbers, and electricians.
Compare the usage of "occupation", which does not imply the exclusivity and special nature of the "professional".
