A state of being ‘on top of it all’.
Inbox Zero is a state in which your Inbox is free from Open Loops. Reaching this state should be the goal of a Weekly Review (if not even a Daily Review). To be at inbox zero, you have to have gone through all of the junk that accumulates and dealt with it - either by doing it (2 Minute Rule of GTD), scheduling it (Contextual Reminders), or organizing it into Project Lists.