Easter egg
Definition
A hidden feature, message, or joke intentionally inserted into a video game, computer program, movie, or other media by its creators, often discoverable only by keen observers.
A real or artificial egg dyed, decorated, or otherwise ornamented, traditionally associated with Easter celebrations and hunts.
Examples
Tucked away in the game's code was an Easter egg that turned all enemies into dancing chickens when activated.
She crafted the most elaborate Easter egg, complete with tiny edible jewels, fooling everyone into thinking it was a Fabergé knockoff.
Movie buffs geeked out over the Easter egg cameo by a long-forgotten actor from the director's first flop.
During the app update, developers slipped in an Easter egg that plays Rick Astley on your birthday.