Professor

A professor is a senior teacher and researcher, usually in a college or university.

Overview

Professors give lectures and seminars in their field of study, such as science or literature. They also do advanced research in their fields and are supposed to do community service (including advisory functions, such as for the government) and train young academics who should replace them. The balance of these four classic fields of professorial tasks depends heavily on the institution, place (country), and time. For example, professors at research universities in the U.S. (and all European universities) are exclusively promoted on the basis of their research achievements.

Differences

The basic difference between types of professor according to national academic system is that in Canada and the United States, the designation is based on career, whereas in Europe, it is based on position. That means that if a North American Assistant Professor is performing particularly well, she or he can be promoted to Associate Professor, and if this is the case again, on to (full) Professor. In the Continental European system, the different fields and sub-fields of teaching and research are allotted certain (professorial) chairs, and one can only become a professor if one is appointed to such a chair (which then has to be free, i.e., unoccupied, of course). Therefore, the different professorial ranks can actually not be compared with each other.

Tenure

A key concept is that of tenure. A professor who holds tenure is virtually indismissable and appointed for life. In theory, professors are free to hold and advance controversial views, as the faculty generally insists on academic freedom. Tenure was thus introduced to preserve academic autonomy and integrity, i.e. the professor was supposed to be kept out of current political or other controversies of the public because it was recognized that this was beneficial for state, society, and academe in the long run. Tenure has recently become under attack by those who want a more business-like approach to universities, including performance review, audits, performance-based salaries, etc.

Survey of the Main Systems and concepts

North American

Main positions:

assistant professor: the entry-level position, for which one usually needs a Ph.D., sometimes only a masters degree (at some colleges). The position is never tenured, although in most institutions, the term is used for "tenure-track" positions; that is, the candidate can become tenured.
associate professor: the mid-level position, usually awarded - in the humanities and social sciences - after the "second book" - although the requirements vary considerably between institutions and departments. Can be tenured or not. In most institutions, the position is tenured, however strictly speaking the position is related to a pay differential and can be awarded to non-tenured persons. If awarded to a non-tenured person, the position is generally tenure-track.
(full) professor: the senior position. Always tenured.

Other positions:

professor emeritus: after full professors retire from active duties, they may continue to teach and to be listed; they also draw a very large percentage of their last salary as pension (as tenure is technically for life). NB: The concept has in some places been watered down to include also associate tenured professors; in some systems and institutions, it needs a special act or vote.
visiting professor: someone visiting another college or university to teach for a limited time; this may be someone who is a professor elsewhere or a distinguished scholar or practitioner who is not.
adjunct professor: someone who does not have a permanent position at the academic institution; this may be someone with a job outside the academic institution teaching courses in a specialized field; or it may refer to persons hired to teach courses on contractual basis (frequently renewable contracts); it is generally a part-time position, although the number of courses taught can vary from a single course to a full-time load (or even an overload); these positions are generally not obligated to participate in administrative responsibilities at the institution often expected of other full-time professors.
named chair: a particularly senior full professor who is awarded a specific, endowed chair that has been sponsored by a fund, a person, etc. Named chairs are usually similar to the Continental European model in that they are a position rather than a career rank.
professor by courtesy: a professor who is primarily and originally associated with one academic department, but has become officially associated with a second department, institute, or program within the university and has assumed a professor's duty in that second department as well. Example: "Henry T. Greely is Professor of Law and Professor, by courtesy, of Genetics at Stanford University". Usually the second courtesy appointment carries with it fewer responsibilities and fewer benefits than a single full appointment.
professor - research: a professor who does not take on all four of the classic duties (see overview) but instead focuses on research. Typically, such a professor may be invaluable to his university department in procuring research funding and/or in publishing scholarly works, and therefore the department would prefer that he not distract himself with teaching duties that are not directly linked to his research activities.

Most other English-speaking countries

Equivalently senior academics to assistant and 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). In that sense, only full professors (North American style) are equivalents of professors

French (France, Belgium)

After the doctorate, scholars who wish to enter academe may apply for a position of maître de conférences ("master of conferences").

After some years in this position, they may take an "habilitation to direct theses" before applying for a position of professeur des universités ("university professor"). In the past, this required a higher doctorate. In some disciplines such as Law and Economics, candidates take the agrégation examination.

German (Central European)

After the doctorate, German scholars who wish to go into academe are supposed to take a Habilitation, i.e. they write a second thesis and spend some time in an inferior position. Once they pass, they are called Privatdozent and are elegible for a call to a chair.

Note that in Germany, there has been always a debate of whether Professor is a title that remains one's own for life once conferred (similar to the doctorate, which becomes part of the legal name), or whether it is linked to a function (or even the designation of a function) and ceases to belong to the holder once she or he quits or retires (except in the usual case of becoming Professor emeritus). The former view has won the day and is by now both the law and majority opinion.

Main positions:

professor ordinarius (ordentlicher Professor, o. Prof.): professor with chair, representing the area in question. In Germany, it's common to call these positions in colloquial use "C4" professorships, due to the name of respective entry in the official salary table for Beamte. (Since the recent reform of the salary system at universities, you might now find the denomination "W3 professor".)
professor extraordinarius (außerordentlicher Professor, ao. Prof.): professor without chair, often in a side-area, or being subordinated to a professor with chair. NB: This is not an associate professor, but rather the equivalent of a full professor; a professor extraordinarius always (with very few exceptions) has tenure, although he is paid less. Often, succesful but junior researchers will first get a position as ao. Prof. and then later try to find an employment as o. Prof. at another university. Colloquially called a "C3 professor" in Germany (or in the new scheme: "W2")
professor emeritus: just like in Northern America (see above); used both for the ordinarius and for the extraordinarius, although strictly speaking only the former is entitled to be called this way. Although retired and being payed a pension instead of a salary, they may still teach and take exams and often still have an office
Juniorprofessor: a very new institution (started in 2003; Germany only), this is a 6-year time-limited professorship for promising young scholars without Habilitation; it is supposed to rejuvenate the professorship through fast-track for the best, who eventually are supposed to become professor ordinarius. This insititution has been introduces as a relacement for the Habilitation (see there for a discussion), which is now considered more an obstacle than a quality control by many. Being new, the concept is highly debated due to the lack of experiences. (The main point of criticism is that the Juniorprofessor is expected to apply for professorships at other universities during the later of the six years, as his university is not supposed to offer him tenure itself (other than in the tenure track schemes used e.g. in the USA).)

Other positions:

Honorarprofessor: equivalent of the North American adjunct professor, non-salaried.
außerplanmäßiger (apl.) Professor: either a tenured university lecturer or Privatdozent to whom the title is given if she or he has not attained a regular professorship after a while, or likewise an adjunct professor. The word außerplanmäßig (meaning "outside of the plan (of positions and salaries)") denotes that he is not paid as a professor but only as a researcher.

Other professors:

Austrian Professor: In Austria, Professor is also an honorific title that can be bestowed upon an artist, scholar, etc., by the President, completely independent from any actual academic assignment.
Gymnasialprofessor (High School Professor): Senior teachers at certain senior high schools in some German states and in Austria were also designated Professor in the late 19th and early 20th century.
Fachhochschulprofessor: professors at a Fachhochschule, which are less paid and don't hold the Habilitation. As they are generally called simply Professor, since the 1970s, professors of any kind that are actually affiliated with a university may call themselves University Professor.

Professors in Fiction

In fiction, in accordance with a stereotype, professors are often depicted as being shy and absent-minded. An obvious example is the 1961 movie The Absent-Minded Professor. See also: mad scientist.

Quote

"Lectures," said McCrimmon, "are our most flexible art form. Any idea, however slight, can be expanded to fill fifty-five minutes; any idea, however great, can be condensed to that time. And if no ideas are available, there can always be discussion. Discussion is the vacuum that fills a vacuum. If no one comes to your lectures or seminars, you can have a workshop and get colleagues involved. They have to come, and your reputation as an adequately popular teacher is saved."

(John Kenneth Galbraith, A Tenured Professor)