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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)