Easter egg

Short definition from Wiktionary

Noun

1. 1
   A dyed or decorated egg, traditionally associated with Easter and, in the Western European tradition, sometimes hidden for children to find.
2. 2
   A chocolate confection in the shape of an egg, sometimes with chocolates or sweets inside.
3. 3
   An undocumented function hidden in a program or video game, typically triggered by a particular input sequence or combination of keystrokes.
4. 4
   Any image, feature, or other content that is hidden on a video disc or in a movie, trailer, or poster.