The meaning of the word professor (Latin: professor, person who professes to be an expert in some art or science, teacher of highest rank) varies. In some English-speaking countries, it refers to a senior academic who holds a departmental chair, especially as head of the department, or a personal chair awarded specifically to that individual. For example, in the United Kingdom, South Africa, Australia and New Zealand it is a legal title conferred by a university denoting the highest academic rank, whereas in the United States, Canada and Hong Kong, the term professor is used as a form of address for any lecturer or researcher employed by a college or university, regardless of rank. In some countries, e.g. Austria, Brazil, France, Romania, Slovenia, Serbia, Poland and Italy, the term is an honorific applied also to secondary level teachers.

Professors are qualified experts, of the various levels described above, who may do the following:

• conduct lectures and seminars in their field of study (i.e., they "profess"), such as the basic fields of science, humanities, social sciences, education, literature or the applied fields of engineering, design, music, medicine, law, or business;
• perform advanced research in their fields.
• provide pro bono community service, including consulting functions (such as advising government and nonprofit organizations);
• teach campus-based or online courses with the help of instructional technology;
• train young or new academics (graduate students).

The balance of these five classic fields of professorial tasks depends heavily on the institution, place (country), and time. For example, professors at highly research-oriented universities in the U.S., and Canada, and, as a general rule, in European universities, are promoted primarily on the basis of their research achievements as well as their success in raising money from sources outside the university.

Islamic usage

In Muslim civilisation, the Chair was designated by the Caliph himself. Mostly through recommendation, the Caliph made appointments to a professorial chair (Kursi in Arabic) in a jami' (university or congregational mosque). Such was the case of Ibn 'Aqil (died 1119 CE) who was appointed to a well-known chair in Jami' al-Mansur (Baghdad), becoming the main teacher of the mosque. In other cases, a scholar could be appointed in two chairs as the same time, holding a chair in one jami' and simultaneously holding another in another jami' or in one of the exclusive institutions.

This is the case of particularly distinguished and popular scholars. For example a certain Ibn al-Banna' (d. 1079) had a chair in Jami' al-Mansur (Baghdad), located in the centre of the riwaq (nave of the mosque); while simultaneously holding another in Jami' al-Qasr (also Baghdad), around the maqsura (a separate room inside the mosque). Some chairs were also known by the discipline they represented; as, for instance, the chair or study-circle of the traditionalists (halqat ahl al-hadith), and that of the grammarians (halqat al-nahwiyin). Others were known by the name of the family whose members occupied it in succession; as, for instance, the chair of the Barmakids (halqat al-Barâmika). Sometimes institutions were specialised in particular study and therefore received a corresponding chair, e.g the Nizamiya did not have a chair of Islamic theology, but only a chair of Islamic law.