Easter egg
The term Easter egg referred to two distinct, but etymologically related, concepts originally based upon a pagan Earth ritual.
Traditional meaning
As a part of the celebration of Easter on Earth, many humans exchanged brightly-colored real or candied chicken eggs, typically by hiding them and then encouraging the recipients to find them. A popular form of candied easter egg was one created of chocolate. The Tenth Doctor partially ate one of these before giving the remainder to Christina de Souza out of concern for its sugary content damaging his teeth. (DW: Planet of the Dead)
DVD technology
With the advent of digital entertainment, it became possible to hide some portions of a recording. Principally employed on DVDs, Easter eggs were not obvious on initial inspection of the recording's menu. On more careful examination, or through the use of sequences entered into the playback device's remote control, these "bonus features" could be found be accessed. Often, the content (or even existence) of the Easter egg would be further obscured when the manufacturer deliberately failed to mention the content on the packaging or in the list of contents.
In 2007, an Easter egg featuring the Doctor was discovered on 14 different DVD releases in Great Britain, leading to some interest among Internet users, including Larry Nightingale, due to the one-sided message's cryptic nature. It was later discovered that the message was recorded in 1969 and addressed to Sally Sparrow, as part of a time travel-related paradox caused by the interference of the Weeping Angels (the 14 DVDs represented Sally's complete collection of discs at that time). Billy Shipton, a former policeman who was transported back to 1969, was responsible for getting the message on the DVDs in such a way that not even the makers of the discs were aware of it. One of the Doctor's statements in the message, "The angels have the phone box", was made into a T-shirt design.
Unknown to anyone except the Doctor, the Easter egg message was encoded in such a way that if any of the DVDs containing it were brought into the TARDIS it activated a prerecorded message identifying it as a control disc, good for a single TARDIS journey. Upon inserting the disc into a DVD drive mounted to the TARDIS' control console, the TARDIS dematerialized and returned to 1969. (DW: Blink)
Behind the scenes
Some Doctor Who discs contain hidden features that are more or less easy to find. These features are usually so small and insignificant that they would not really be appropriate for inclusion on the main disc menu. However, disc producers sometimes like to sneak these little gems onto the disc in "hidden" places. Examples have included hidden blooper reels and interviews. American filmmaker Kevin Smith and the makers of the first American Pie movie used Easter Eggs to poke fun at their audience for looking for Easter Eggs.
When the 2007 season of Doctor Who was released to DVD, the Blink Easter egg message was, appropriately, included in the set as an Easter egg.
