Easter Egg

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.

Usually, such Easter Eggs come in the form of a hidden character, scene or reference to something else outside the subjects area (for example a tribute to a series that is hidden in a secret area). Due to legal reasons, many companies invoked strict rules about Easter Eggs, not only for copyright reasons but also due to the problems having something in the game that has not been announced openly.

The game Atari's Adventure, released in 1978, contained what is thought to be the first video game Easter egg (the programmer, Warren Robinett's name) ever produced.

One Piece
Oda is renouned for featuring many Easter Eggs in One Piece, the most notable and famous being Pandaman.