Lynxy and phrack50
An Easter Egg refers to something hidden in a program. It differs from a cheat in that
a) it doesn't have to be in a game, and
b) it usually doesn't actually effect the playing of the game
For example, there was a way, if you went to a specific cell in Excel 97, and pressed a specific combination of keys, that a simple flight simultor would start.
From dictionary.com
http://dictionary.reference.com/search?q=easter egg
"<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 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!"
hehe, cool, it used the same example as me. Hope that explains it for you Lynxy. And I didn't even have to be negative about it.
*EDIT*
oh, and I meant to say, in relation to maps, it usually means, as Madd Dog said, a hidden room, which usually contains the sig or picture or name of the map maker. This room is sometimes accessible in the game, and sometimes only via a map editor.