Easter egg

a chicken egg that is dyed and often given a figure or design, or an imitation of such an egg, as an egg-shaped candy or chocolate, used at Easter as a gift or decoration.

Digital Technology. an extra feature, as a message or video, hidden in a software program, video game, DVD, etc., and revealed as by an obscure sequence of keystrokes or commands.

Movies, Television. a hidden message, as a cryptic reference, iconic image, or inside joke, that fans are intended to discover in a television show or movie.