The term professional physicist has multiple meanings. The most widely used meaning is that it is a physicist who works in physics for a living. This meaning would include theoretical physicists and experimental physicists, and may also include physics professors and authors of physics books and textbooks.
The term may also refer a physicist who has received professional certification. In particular, the Canadian Association of Physicists offers a certification exam and has registered "Professional Physicist" as a trademark in Canada. The Institute of Physics in the United Kingdom grants a similar title, "Chartered Physicist".