Emeritus professor
Emeritus is the title given to a professor who is retired. The term is also used for other occupations, most notably clergy, and presidents. Professors who are emeriti no longer need to fulfill all of their functions (such as teaching at a university), but they may still do so. Pope Benedict XVI retired on  11 February 2013, and was the first pope to do this in recent history. Today, he is Pope Emeritus. Benedict XVI was a professor at the University of Regensburg.