Easter egg

The term Easter egg referred to two distinct concepts related to a part of the celebration of the holiday of Easter on Earth.

Traditional Meaning
As a part of Easter, many humans exchanged brightly-coloured chicken eggs or egg-shaped candy, typically by hiding them and then encouraging the recipients to find them.

A popular form of confectioneries Easter egg was one created of chocolate and wrapped in coloured tin foil. The 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 Feature
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 17 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 17 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, "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 (or possibly a specific DVD, if Sparrow later indicated to the Doctor which film it was) 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

 * In episode 13 of the fourth series of Doctor Who Confidential, actor Billie Piper revealed that chocolate Easter eggs plagued the filming of the finale of Series 4. According to her, the cast of the interior TARDIS scene — during which Earth gets towed home — had consumed copious quantities of eggs during filming.  As a result, many of the actors were on a sugar rush, and therefore prone to bouts of hyperactivity and giggling.
 * Many classic-series Doctor Who DVD releases contain Easter Egg content, usually accessible by highlighting hidden areas on menu screens or, in a couple of cases, by letting an episode play to the very end. Examples of this content have included outtakes, interview clips, and continuity announcements. Some of the more unique Easter eggs have included a sample of a scene remastered via the VidFIRE technique included with The Tomb of the Cybermen, a hidden commentary track with David Tennant included with the Special Edition release of The Five Doctors, and a short tribute to Anthony Ainley (consisting of an outtake from the Destiny of the Doctors computer game) which was appended the end of episode 4 of The Keeper of Traken.
 * 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.