Doctor

Doctor was a title used by physicians and the holders of certain advanced university degrees. According to River Song, the word came from the renegade Time Lord known as the Doctor. While the term originally meant "healer" or "wise man," some cultures used the word to mean "great warrior" due to their fear of the Time Lord known as the Doctor. It is implied that these cultures had forgotten the word's true origin. (TV: A Good Man Goes to War)

Behind the scenes

 * The idea that the word "doctor" derives from the Doctor was first advanced by Steven Moffat in a Usenet post in 1995.