∂³Σx²

According to the secret files created by the High Court of the Time Lords, the Doctor's true name was a mathematical formula: ∂³Σx². (PROSE: The Trial of Doctor Who)

The Old High Gallifreyan word was also inscribed in the Tomb of Rassilon, (TV: The Five Doctors) and could be seen on K9 Mark I's regeneration unit. (TV: Regeneration)

One account, which also depicted the High Court of the Time Lords, indicated that the Doctor's true name held a mystic importance that caused him to keep it secret. It was also unpronounceable to humans. This was true for all Time Lords and their names. (PROSE: Doctor Who and the Auton Invasion)

Behind the scenes

 * PROSE: The Trial of Doctor Who is the only valid story, as this Wiki defines it, that explicitly confirms that "∂³Σx²" is the Doctor's true name.
 * However, this name has not been widely accepted. The Doctor's name is a contentious subject with aplenty of conflicting names and aliases, including "Theta Sigma", "Doctor Who", and simply "The Doctor". 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 absolutely nothing.
 * It is technically possible to pronounce each character in the name: can be dee/doh/die, ³ is cubed,  is sigma, x is ex, and ² is squared, so therefore a pronunciation of the name could be dee-cubed-sigma-ex-squared, though this is unlikely.
 * Theta Sigma (ΘΣ) is given as the Doctor's name in TV: The Armageddon Factor. This is retconned to be the Doctor's "nickname at college" in TV: The Happiness Patrol. The names ∂³Σx² and ΘΣ both contain the Greek letter sigma (Σ).
 * The Doctor's real name was spelled as "d³ᓬx²" in Marvel Premiere #57. Due to the non-narrative structure of that source, this 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 installed it.