Easter egg  
noun [ C ] uk/ˈiː.stər ˌeɡ/ us/ˈiː.stɚ ˌeɡ/
  (EGG)       
  复活节巧克力蛋
chocolate in the shape of an egg, given as a present or hidden for children to find at Easter
  mainly US复活节彩蛋 
an egg that is dyed (= coloured) and sometimes decorated, and hidden for children to find at Easter
   (SURPRISE)      
 复活节彩蛋(在电脑游戏、软件或电影等中隐藏的惊喜或额外的功能)
a hidden surprise or extra feature that is included in something such as a computer game, a piece of software, or a film, for the person using or watching it to find and enjoy
  How can I find out whether any Easter eggs are hidden in my software?  我怎样才能知道我的软件中是否藏有复活节彩蛋? 
 - More examples
 - Easter eggs are little bonuses hidden in anything from computer programmes to films.
 - You need to tap everywhere on the screen since there are Easter eggs to be found.
 - You could play forever without finding every Easter Egg in this virtual universe.
 - The DVD includes a blooper reel, as well as an Easter egg in which the director enlists Willem Dafoe to prank Alfred Molina.