Ground Zero, or 0n 0e, is a term used to describe the central location of every world.
Ground Zero in Worlds
The world ground zero is most commonly used as an entry point, even when the entry point isn't located at the exact centre, the ground zero terminology is often times used. The ground zero of a world would commonly be decorated with information areas and other things to service the users need, keep them occupied with games such as tic tac toe and draw them in to build in the world.
Ground Zero in cities
Ground zero can also refer to the point of entry into cities, as with Worlds, they are usually surrounded by a lavishly decorated information area, or something to grab the users attention. This also complies with the original idea that Active Worlds be used for 3D websites where one makes a highly decorated main page to grab users attention.
Often these ground zeroes will conform to a theme, such as moonlight heights Forbidden City.