General: easter egg
An Easter egg is a hard-boiled egg that is decorated to celebrate the Easter holiday. This term can also refer to colorful, plastic, egg-shaped containers that are usually filled with small objects like candy. On Easter, eggs are hidden throughout fields (supposedly by the "Easter bunny") for children to go on Easter egg hunts.
An Easter egg also refers to a hidden feature, message, or inside joke inserted in digital software (computer programs, video games, DVD menus, et cetera) and hardware for users to find. The term, which originated from a hidden message in the Atari 2600 video game Adventure, is meant to invoke the idea of a traditional Easter egg hunt. These kinds of Easter eggs can be invoked on e621's images using notes (although we strongly encourage you not to abuse this feature). For example, see if you can find the Easter egg hidden in post #1338151.