A network segment between an external and an internal FireWall, where exposed hosts can be placed that are accessible from the external InterNet as well as from the internal network. These hosts are treated by the inner FireWall as anyone else from the InterNet: they're blocked from access to the inner network. This way, potential corruption of endangered public services such as a WebServer does not threaten the integrity of the internal network.
Untrusted local networks are often also connected to the DMZ. Examples of such networks include WirelessNetworks, PublicAccessNetworks, student labs or networks set aside for cleaning Viruses off machines.