Easter egg
Easter egg: [from the custom of the Easter Egg hunt observed in
the U.S. and many parts of Europe] n. 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, or
sound effect emitted by a program (or, on a 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 OSes caused them to respond
to the command `make love' with `not war?'. Many
personal computers have much more elaborate eggs hidden in ROM,
including lists of the developers' names, political exhortations,
snatches of music, and (in one case) graphics images of the entire
development team.