Professor is a title given throughtout history to very learned people teaching to others usually at a university.
20th century
Ace referred to the Doctor as Professor. [source needed]
51st century
River Song introduces herself as a Professor at a University of Archaeology. (DW: Silence in the Library)
100 trillion
In the year 100,000,000,000,000, Yana was given the title of Professor even their had been no universities for a century. (DW: Utopia)