Easter egg

An Easter egg is a feature hidden within a program, usually intended to be found for humorous purposes by persons using it. It may be as simple as a message hidden within the object code of a program, or a more elaborate graphic, sound effect, or other behavior emitted by the software in response to some undocumented set of commands or keystrokes, to be discovered as a surprise joke or to display program credits.

History
One early Easter egg found in some operating systems caused them to respond to the command "make love" with "not war?". Many personal computers, and even satellite control computers, have more elaborate eggs hidden in ROM (such as the of an IBM PC), including lists of the developers' names (e.g. Microsoft Windows 3.1x), political exhortations and snatches of music. The (CoCo) had images of the entire development team. Microsoft Excel 97 includes a flight simulator.

Apple has long been known to have easter eggs within their products. Steve Jobs instituted a ban on Easter eggs when he returned to Apple in the late 1990s, though it did not last very long.

Articles

 * 16 Easter Eggs Hidden in Apple Products by Rudie Obias at Mental Floss (2016-03-21)
 * 6 hidden easter eggs in your iPhone you probably didn’t know about by Sofia Svantesson at Soluno (2020-10-16)