Zero Infrastructure or ZI is a harsh military action in which the defeated nation's infrastructure is completely destroyed. Since national economies in the Cyberverse rely on infrastructure, being "ZI'ed" is the CN equivalent of being "knocked into the Stone Age."
Probably the most accurate description of the different methods of ZI was given by Londo Mollari of Athens on 4 February 2009:
Sometimes if you cause significant damage to an alliance, spy on an alliance, go rogue on an alliance, engage in excessive flaming, commit OOC attacks, etc.... the alliance's leadership will decide to respond by placing an edict of ZI upon you. ZI stands for zero infrastructure, and it means that you will be attacked by their alliance until you have no infrastructure left. There are three kinds of ZI - single ZI, permanent ZI, and eternal ZI. A single ZI means that you will be attacked until you have no infrastructure left, then left alone and allowed to rebuild. A permanent ZI, or PZI, means that you will be attacked until the alliance which is attacking you decides to quit. PZI sentences have been known to last years in some cases, but they usually end one way or another. Eternal ZI means that any nation you create, even if you delete your first nation, will be attacked forever if your enemies catch you.