Easter egg

A virtual Easter egg is an intentional hidden message, in-joke or feature in a work such as a computer program, web page, video game, movie, book or crossword. The term was coined—according to Warren Robinett—by Atari after they were pointed to the secret message left by Robinett in the game Adventure.^[1] It draws a parallel with the custom of the Easter egg hunt observed in many Western nations as well as the last Russian imperial family's tradition of giving elaborately jeweled egg-shaped creations by Carl Fabergé which contained hidden surprises.^[2]
This practice is similar in some respects to hidden signature motifs such as Diego Rivera including himself in his murals, Alfred Hitchcock's legendary cameo appearances, and various "Hidden Mickeys" that can be found throughout the various Disney Parks. An early example of this kind of "Easter egg" is Al Hirschfeld's "Nina".
Atari's Adventure, released in 1979, contained what was thought to be the first video game "Easter egg", the name of the programmer (Warren Robinett). However, evidence of earlier Easter eggs has since surfaced. Several cartridges for the Fairchild Channel F include previously unknown Easter eggs, programmed by Michael Glass and Brad Reid-Selth, that are believed to predate Robinett's work.
Software-based
Easter eggs are messages, videos, graphics, sound effects, or an unusual change in program behavior that sometimes occur in a software program in response to some undocumented set of commands, mouse clicks, keystrokes or other stimuli intended as a joke or to display program credits.
Non-software
While computer-related Easter eggs are often found in software, occasionally they exist in hardware or firmware of certain devices. On some home computers, the BIOS ROM contains Easter eggs. Notable examples include several early Apple Macintosh models which had pictures of the development team in the ROM (accessible by pressing the programmer's switch and jumping to a specific memory address, or other equally obscure means), and some errant 1993 AMI BIOS that on 13 November proceeded to play "Happy Birthday" via the PC speaker over and over again instead of booting.^[12] Similarly, the Radio Shack Color Computer 3's ROM contained code which would display the likenesses of three Microware developers on a Ctrl+Alt+Reset keypress sequence—a hard reset which would discard any information currently in the dynamic memory.