easter egg

A dyed or decorated egg, traditionally associated with Easter and, in the Western European tradition, sometimes hidden for children to find.

A chocolate confection in the shape of an egg, sometimes with chocolates or sweets inside.

(computing, video games) An undocumented function hidden in a program or video game, typically triggered by a particular input sequence or combination of keystrokes.

(by extension) Any image, feature, or other content that is hidden on a video disc or in a movie, trailer, or poster.