The Uncle

 was a Time Lord from Gallifrey and an uncle of the Doctor.

While his mind was within the time brain, the Seventh Doctor, in response to Albert Einstein's statement, "It is a fundamental postulate that all motion is relative", told him that Einstein would not say that if he had met his uncle. (TV: Time and the Rani)

On one occasion, River Song used the British idiom "Bob's your uncle" in front of the Eleventh Doctor, who retorted that his uncle was not called Bob, but "the Uncle." When River expressed skepticism about the existence of such a name, the Doctor described it as "a Time Lord thing." (GAME: The Eternity Clock)