An Easter Egg is something not really map related that a map maker intentionally built in. A well known example is the (partly incorrect) lecture about the word "fuck" that can be triggered in the "Highrise" map. This speech sample has nothing to do with the map or the gameplay, but it's an entertainment factor.
The way an Easter Egg is visited or triggered is part of the Easter Egg fun. In "Highrise", you have to find a certain spot that you'd usually not visit. In another map, a decoration might secretly behave as a push- or shoot-button. Or there might be a wall that you can walk through.
It can be disputed if an Easter Egg area that people can actually really build/play in is still an Easter Egg. Even if its theme is drastically different from the rest of the map, it's technically a regular game area.