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 Doctor. It is implied that these cultures had forgotten the word's true origin. (TV: A Good Man Goes to War)

The Doctor as a medical doctor
In some circumstances, the Doctor pretends that his title is to be intended as a physician.

First Doctor

 * With the pirate Cherub and his party, who called him "sawbones". (TV: The Smugglers)

Second Doctor

 * With Kirsty McLaren and her sick father, in Scotland, in 1746. (TV: The Highlanders)
 * To enter and rummage the Medical Centre of the Gatwick Airport, in 1966, along with the fake patient Jamie McCrimmon. (TV: The Faceless Ones)

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.