In Canada and the United States, the faculty of a college or university is organized hierarchically, ranging from teaching assistants, assistant professors, associate professors, to full professors. In most other English-speaking countries, equivalently senior academics to assistant professors and less-prestigious associate professors are generally known as "Lecturers", "Senior Lecturers" and "Readers", with professorships reserved for only the most senior academic staff. A Professor in these countries holds either a departmental chair (generally as the head of the department or of a sub-department) or a personal chair (a professorship awarded specifically to that individual).
Professors give lectures in their field of study, such as science or literature. They also do advanced research in their fields. Many Nobel Prize laureates are professors. After a professor retires from active teaching duties, they often continue to appear on faculty listings, designated as professor emeritus.
In theory, professors are free to hold and advance controversial views, as the faculty generally insists on academic freedom. Full professors are usually awarded "tenure". Generally, a tenured professor cannot be sacked except in the case of gross misconduct.
A visiting professor is a professor visiting another college or university to teach for a limited time.