Easter egg

From Styleguide

A hidden message, object or feature found in a game that is generally unnecessary, unrelated and otherwise outside of the course of normal gameplay. Common examples of Easter eggs include messages from game programmers to fans and relatives, pictures of development teams and inside jokes. Coined by Atari's Steve Wright.

Example: Warren Robinette's hidden initials in Adventure for the Atari 2600 constituted the first instance of an Easter egg in a videogame.

Wrong: easter egg, Easter Egg.

Also see: code, unlockable.