When referring to media, an Easter egg is something hidden by the creators for observant fans to find, much like a child finding a hidden Easter egg. Common forms of Easter eggs are the developer's names, hidden pictures, and in-jokes. The term was coined in 1979 by Steve Wright, an employee of Atari, who hinted that upcoming games would contain them. Although the term originated with video games, it has since spread to all other forms of media. Many different sources claim the 1980 game Adventure was the first video game to feature an Easter egg, but they existed in several earlier games.
Unless the media's creator explicitly identifies content as an Easter egg, which is rare because it ruins the fun of finding them, it's difficult to be sure what content is meant to be an Easter egg as opposed to just a joke, nod to the player, or similar occurrence. When the intention of the creator is unknown, the following criteria can help identify a genuine Easter egg:
The point of an Easter egg hunt is the fun we get from finding something which is hidden, and this is the same with Easter eggs in media. Should it count as an Easter egg if it stands out so much that everyone sees it? For example, in Disney's Hercules, there is a scene where Hercules is wearing a lion skin which is very similar looking to the character Scar from The Lion King. This was clearly added by the animators to reference to an earlier film, and, while some web sites list this as an Easter egg, the lion skin is on the screen for several seconds, so it's pretty much impossible to miss. Contrast this with a scene from Beauty and the Beast where Maurice is trying to find his way in a dark forest. He shines his lantern on a sign post for only a second and the writing is so weathered it's barely decipherable. Even if you freeze-frame the scene, it's hard to make out the words which read "Anaheim" and "Valencia," two cities important to the animators. These are hidden much better, so it's makes more sense to refer to this as an Easter egg.
Of course, "hidden" is a matter of opinion, some people are more observant than others, or in different ways, so, one person may consider something hidden which another will consider glaringly obvious. Also, simply being hidden isn't enough to call something an Easter egg. In media, it's typical to find things which could be considered "hidden" simply because they aren't conspicuous. This isn't uncommon in the backgrounds of video, but entirely expected in interactive media like video games, especially in certain gaming genres like adventure which use hidden things as a mechanic.
Out of Place
One way to help distinguish between a genuine Easter egg and something that's just hidden is to consider if it fits the game's theme. For example, in the fantasy Ultima VII: The Black Gate, you can find a clue which gives you map coordinates. Following them with a sextant leads you to small island with a cave filled with pirate treasure. It's certainly hidden, but it also completely fits the game's theme, so it doesn't make sense to call it an Easter egg. Contrast that with Myth: The Fallen Lord, also set in a fantasy world, but, if you go to an unmarked area in the second mission, you can find a redneck living in a trailer with country music playing and empty beer bottles littered around his front yard. This clearly doesn't belong in the game's setting, so it's far more likely this was intended to be an Easter egg.
"Out of place," like "hidden," is also a matter of opinion. This is especially true in comedic or satire games. The Space Quest series, for example, has a large number of references to outside books, films, and video games, so many that you could argue that all of the content in every game fits with the theme. However, the developers still managed to sneak in some Easter eggs which don't fit the theme. For example, in early versions of the first game, typing "ken" in the opening room would cause Ken Williams, the boss of the game's developers, to walk out and complain about the game being behind schedule. A dig at one's boss isn't a spoof on another franchise, so it's probably fair to say it is out of place, even in a satire game.
This criteria isn't as important, but it does matter to some people. Many video games contain hidden aspects as a mechanic, and some of those hidden aspects may also be out of place, but should they count as Easter eggs if finding them is necessary to finish the game? For example, after the player gets through all the puzzles of Space Quest III: The Pirates of Pestulon they will meet Ken Williams, CEO of Sierra On-Line. However, while meeting Ken Williams is generally accepted as a genuine Easter egg in Space Quest I: The Sarien Encounter, few accept his inclusion in the third game as an Easter egg. Like in the first game, the segment is out of place, and also "hidden" in the sense it isn't available to the player until they solve a series of puzzles, so why do so few people count it as an Easter egg? The difference seems to be based on the fact that it is a necessary part of the game's story; the player can't beat the game without doing this. Although this may sound a bit arbitrary, the origin of Easter eggs help clarify it. Most early Easter eggs were the result of clever programmers sneaky unnecessary additions into games either as a joke or to stick it to their bosses. Because of this, it doesn't really feel like an Easter egg when it's a mandatory part of a game's main plot.
Part of the Media
This is more of a distinction than a criteria. Digital media can be experienced in its normal manner (e.g., playing a game, watching a film, listening to music, etc.), or in its encoded manner, that is, viewing the series of ones and zeroes which make up the digital data.
The creators of digital media frequently leave messages in the encoded portions of the data which can never be seen by anyone experiencing the media in its normal manner. For example, if you open the files of the MS-DOS port of Silpheed: Super Dogfighter in a hex editor you will find various messages, in one, a person memorialized a wedding. You can never see this message while playing the game, it was written by one of the game's developers knowing that it would only be seen by hackers. Notes like this are quite common and are referred to by the gaming community as "developer messages." They are certainly similar to Easter eggs, in the sense that they're hidden, out of place, and optional, but they exist more as a way for the developers to communicate to other technically minded people rather than a general audience. Because of this, not everyone agrees that they should be considered genuine Easter eggs.
|Adventure||1980-??-??||Find an invisible dot in the black castle then use it to walk through a wall to see the developer's name. Often incorrectly labeled the first Easter egg, but it's the first in a popular game.|
|Alien Invasion||1981-??-??||Start game 5, kill player 1 without firing, have player 2 kill the lowest row of aliens, then kill players 1 and 2 without firing, then restart game 5. The programmer's last name will become visible and the UFO will turn into a duck as long as you don't shoot.|
|Moonlander||1973-??-??||Travel several screens over in the zoomed in mode then land safely or crash into to the MacDonald's. Not necessarily and Easter egg since it's not all that well hidden.|
|Spitfire||1977-??-??||Enter a long string of numbers on the console at the game's start screen to see the developer's name.|
|Starship 1||1977-07-??||Hold down both fire buttons and press start, let go, then hit the "Slow" lever to see the developer's name.|
|Video Whizball||1978-??-??||Restart game 4-3 with a score of 67 to see the developer's name.|
|Yars' Revenge||1982-05-??||Hit the swirl mid-flight, then move your Yar to the line on the lower third of the screen and wait. When the explosion ends, you will see the developer's initials and the game will end.|
- eeggs.com - The Easter Egg Archive - online database of Easter eggs in media.
- digitpress.com/eastereggs - Easter eggs and bugs in video games.