∂³Σx²
According to the secret files created by High Court of the Time Lords, the Doctor's true name was ∂³Σx²; (PROSE: The Trial of Doctor Who) indeed, the name was inscribed upon K9 Mark 1's regeneration unit. (TV: Regeneration)
Curiously, the name was also seen in the Tomb of Rassilon, perhaps an example of Old High Gallifreyan, the archaic language used in the Time of Rassilon. (TV: The Five Doctors)
Behind the scenes
PROSE: The Trial of Doctor Who is the only valid story, as the Wiki defines it, that explicitly confirms that "∂³Σx²" is the Doctor's true name. The claim was also reiterated in Marvel Premiere 57, though due to the non-narrativity of that source, the Wiki does not acknowledge it as part of the DWU.
In TV: Regeneration, the regeneration unit that the name is inscribed upon is implicitly linked to the Doctor, with hints that the Doctor was the one who sent it.
In TV: The Five Doctors, the presence of the is somewhat odd due to it being presumably an ancient carving. One could speculate links to the Other or the Timeless Children.
However, this name has certainly not been widely accepted, as the Doctor's name is a contentious subject with aplenty of conflicting "names", such as "Doctor Who", "Theta Sigma", and "The Doctor" being some notable examples. Moreover, despite the few sources revealing that "∂³Σx²" is the Doctor's name, how to pronounce it, or even the relevancy of it, is undefined. So it simultaneously answers one of the largest questions in Doctor Who while also resolving nothing.
