<jargon> (From the custom of the Easter Egg hunt observed in the US and many parts of Europe)
1. A message hidden in the object code of a program as a joke, intended to be found by persons disassembling or browsing the code.
2. A message, graphic, sound effect, or other behaviour emitted by a program (or, on an IBM PC, the BIOS ROM) in response to some undocumented set of commands or keystrokes, intended as a joke or to display program credits.
One well-known early Easter egg found in a couple of operating systems caused them to respond to the command "make love" with "not war?". Many personal computers, and even satellite control computers, have much more elaborate eggs hidden in ROM, including lists of the developers' names (e.g. Microsoft Windows 3.1x), political exhortations and snatches of music. The Tandy Color Computer 3 (CoCo) had images of the entire development team. Microsoft Excel 97 includes a flight simulator!