Sigma

Sigma may refer to:
 * Ood Sigma, an Ood (TV: Planet of the Ood)
 * Sigma (planet), the home of Dr. Ivan Asimoff (DWM: "The Fabulous Idiot")
 * Earl Sigma (TV: The Happiness Patrol)
 * Theta Sigma, an alias of the Doctor (TV: The Armageddon Factor, The Happiness Patrol, COMIC: Flashback)