An Easter Egg is a hidden suprise in software and some content types.
A virtual Easter egg is a hidden message or feature in an object such as a movie, book, CD, DVD, computer program, or video game. The term draws a parallel with the custom of the Easter egg hunt observed in many western nations. The origin of the term is sometimes falsely attributed to the movie Return of the Living Dead, where a military officer uses it as a code word for lost U.S. government containers of zombies created by a chemical spill, or to The Rocky Horror Picture Show, in which actual Easter eggs are visible in certain shots (under Frank N. Furter's throne, for example). Return of the Living Dead was not released until 1985, and Atari's Adventure, released in 1978, contained what is thought to be the first video game Easter egg (the programmer, Warren Robinett's name).
In computer programming, the underlying motivation is probably to put an individual, almost artistic touch on an intellectual product which is by its nature standardised and functional, although Warren Robinett's motivation was more likely to gain recognition, since video game programmers were routinely uncredited then. It is analogous to signature motifs such as Diego Rivera including himself in his murals or Alfred Hitchcock including himself in the opening scenes of his movies (the latter known as a cameo).
There is an Architecutral Easter Egg in NYC.
There is a building with 2 revolving doors at its entrance. There is an enclosed space space between the doors. As you pass through the doors, if you are a keen observer (or someone shows you this) there is a slit, about 9 inces wide, viewable from the revolving door as you pass through it. The slit peeks into the enclosed space between the doors in which there is a sculture, about 3/4 life-size of a naked girl. It is lite from above and rotates slowly.
The story I was told is that the building was once either a Playboy Club or Playboy offices in NYC. When playboy left the new new owners of the building enclosed it, but left it as a homage to its history. I have not varified this.
And I am not going to tell you where it is. If you want to see it, you'll have to do some hunting.