A true Easter Egg must satisfy the following criteria:
1. Undocumented, Hidden, and Non-Obvious
An Easter Egg can't be a legitimate feature of a product, or be an obvious part of a storyline. Easter Eggs will usually stand out either because they totally don't fit with their context (like a pinball game in a word processor), or because they have a deeper hidden personal meaning to the creators, so they threw it in for entertainment.
Every user with the same product or combination of products must be able to produce the same result given the instructions. If others can't reproduce an Egg, then it doesn't belong in this archive.
3. Put There by the Creators for Personal Reasons
The Egg must have been put there on purpose, and furthermore have a personal significance to the creators beyond just making a better product (movie, TV show, software program, etc).
4. Not Malicious
Easter Eggs are there for fun, not to do damage.
The most important element... if it's not there for entertainment, it's not an Egg.
Have a look at Easter Egg Archive