easter egg (meta)
An Easter egg can refer 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.
These should not be mistaken for the traditional sort of Easter eggs which don't come with the _(meta) suffix.