The Time of the Doctor
In "The Wedding of River Song" (2011), it is implied that the Doctor's true name is a secret that must never be revealed; this is explored further in "The Name of the Doctor" (2013), when River Song speaking his name allows the Great Intelligence to enter his tomb, and in "The Time of the Doctor" (2013) where speaking his true name becomes the signal by which the Time Lords would know they can safely return to the universe.