Bad Wolf
The term "Bad Wolf" appears in every episode of the first series (except for the two-part stories which only have one explicit reference between the two episodes) between "The End of the World" and "The Parting of the Ways", in which the mystery is cleared up. (See Story arcs in Doctor Who.) It is also seen in further series of Doctor Who and in the Torchwood episode "Captain Jack Harkness" as graffiti inside the dance hall.
References to "Bad Wolf" outside the series include its use as a password for the UNIT website,[1] several appearances on the BBC's own web site,[2] and its use in the first three of the New Series Adventures spin-off novels. The BBC Bad Wolf website lists the various clues and possible theories as to its identity.[3]
Upon revealing the Bad Wolf Corporation logo to the Doctor, Lynda proclaims the line "Your lords and masters" - a line echoed in "Daleks in Manhatten" by Eric Loren's character Mr. Diagoras.
In the Fourth Series episode Turn Left, the term "Bad Wolf" is sent as a message to the Doctor by Rose Tyler, through Donna Noble, to signify the end of the universe.