An Easter Egg is something that cannot easily be found.
Usually the reward for finding it is a bit more interesting than plain text.

As we are not little children, we are supposed to hide Easter Eggs and not find them. 
So an Easter Egg is something that shows up, when the user of a program performs a predefined action. 

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Term coined by an Atari executive upon discovering WarrenRobinett's secret room in AtariAdventure, considered by most to be the first Easter Egg.

'''Objection.''' This term has been used by technicians for ''at least'' thirty years that I know of. If one wants to find a defective board in a rack one swaps them out one at a time. When the failed board is replaced, the rack comes alive. This was referred to as "Easter Egging" for a fault. The same idea applied to any kind of Easter Egg hunt for some solution when the hunter knew not his stuff. Duh! -- MartySchrader

I think "Easter Egging" has been done at the Easter time for even longer than that. :-) The term in trouble hunting probably has the long history stated above and in computer program's HiddenFeatures the history stated above that. -- TomiBgtMantyla

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Try http://www.eeggs.com
-- LeeNathan
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See HiddenFeatures.