Easter egg
n//ˈiː.stəɹ ˌɛɡ//påskägg
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. [from 1980s]
(by extension) Any image, feature, or other content that is hidden on a video disc or in a movie, trailer, or poster.