Easter egg

From IFWiki

In general, an amusing message or image hidden in a computer program until displayed in response to an obscure user action. Historically, examples have included lists of all the programmers' names and humorous responses to the command XYZZY.

In IF, the term is often applied loosely to optional interactions that do not significantly affect the plot but are nevertheless aesthetically rewarding.

Named for the custom of hiding decorated or candy eggs on Easter Sunday.