An easter egg is a message where it refers to the developer's influences, inside jokes, etc. Usually it's hidden and not directly seen by the player.
Console easter eggs Edit
Typing some of Doom's cheat codes in the console displays a special message.
|iddqd||"College Degreelessness Mode ON"|
|idkfa||"That is indeed one killer fucking arsenal you have there."|
|idspispopd||"I'll smash your FACE into small piles of putrid debris!"|
|idchoppers||"Get to da choppah!"|