Theta

Theta was a letter in the Greek language alphabet of Earth, originating in the early period of human history.
It appeared in the following contexts:
* Theta Sigma, a nickname for the Doctor (TV: The Armageddon Factor)
* Theta Sigma (Forever), an alternate version of the Doctor (AUDIO: Forever)
* "Omega, Sigma, Theta, Ohm", the voice-activated termination code for the Korven Space-Time Manipulator (TV: The Eclipse of the Korven)
* Theta Stigma, an alternate version of the Eighth Doctor. (COMIC: The Glorious Dead)