X

As spoken by Sarah Jane Smith, X was the twenty-fourth letter of the alphabet, following W and preceding Y. (TV: The Masque of Mandragora, Goodbye, Sarah Jane Smith)

X was the Space Service security designation for the Dalek Inquisitor General during the Second Dalek War. This earned the incumbent Inquisitor General the nickname of Dalek X, a name which the Inquisitor General himself soon adopted. (PROSE: Prisoner of the Daleks)