What are unique job titles in academia called? Occasionally a university gives a famous professor a job title individually designed for that one person only.  Thus:


Donald Knuth is "Professor Emeritus of the Art of Computer Programming" at Stanford.
Gian-Carlo Rota was "Professor of Applied Mathematics and Philosophy" at M.I.T., although he had an appointment only in the mathematics department.


Is there a common noun referring to such custom-designed job titles for rare famous professors?

PS inspired by things written below: This is not the same as an endowed chair or a distinguished professorship.  Some universities use the titled of "Distinguished Professor" and have rules saying only a limited number of professors can have that title; some universitites call the appointees "University Professors"; M.I.T. calls them "Institute Professors" (Isadore Singer in the math department is an Institute Professor). But with Distinguished Professors there is some codified rule saying these can exist and setting forth certain criteria for granting such a title.  Similarly for endowed chairs.  Knuth's title was based on the name of an influential and famous three-volume work on computer programming that Knuth had written; it would be absurd to promulgate a rule saying who might be appointed to this position in the future and according to which criteria, since it's a title that can fit only one person.  Nor could it have been contemplated in Rota's case that others might some day have that title; it was awarded only because of the unique circumstances of Rota's life.
In the United States, these types of "one-off" titles are typically given as a stage of promotion beyond full professorship.  The typical generic name would be "chaired professor," "distinguished professor," or "titled professor."  This is a rank that many faculty never reach, and typically the highest rank that a professor can achieve in the university as a professor---any further "promotion" is actually a change of position into management, e.g., dean, provost, president.

Such positions often come with a set of funds dedicated to support them, giving the faculty promoted to the position additional freedom of operation.  Often the exact title is set by the person or organization that endows support for the position.  For example, my thesis advisor is the "Panasonic Professor of Electrical Engineering" at MIT, meaning that his particular "one-off" position was endowed by the Panasonic corporation.  Such ties, however, typically mean little besides the name.