The Zone is a part of the gaming nexus.
The Zone occurs whenever a human being gets drawn too much into a game of choice. They begin to mix reality with fiction and soon the fiction overrides the reality, causing the line between the two to break.
The first visit to "the zone" was done by Greg Dean as he played Ultima Online. Attempts to remove him from his computer failed, including life-threatening warnings and smacks to the head. He believed himself to be Sagus for a long time, until Crystal kissed him, removing him from the world.