Professor

Professor was a title given to scholarly teachers to denote their academic ability. It was often, though not exclusively, given to those who had achieved a certain level of learning of held a position that earned them the title.

Ace often referred to the Seventh Doctor as "the Professor" throughout her early association with him (TV: Dragonfire) and still continued to refer to him as 'professor' in a friendly manner several years later. (PROSE: Happy Endings) This may have been in reference to the fictional character Professor X.