Theta

Theta was a letter in the Greek language alphabet of Earth, originating in the early period of Human history.

It was used in the following contexts:


 * Theta Sigma, a nickname for the Doctor (DW: The Armageddon Factor)
 * Theta Sigma (Forever), an alternate version of the Doctor (BFG: Forever)
 * 'Omega, Sigma, Theta, Ohm', the voice-activated termination code for the Korven Space-Time Manipulator (K9TV: The Eclipse of the Korven)