A computer system physically isolated from unsecured networks is described as an air gap.
Air gapping is a network-security measure in which a computer or network has no connected wired or wireless interface to an outside network, such as the public internet. The name compares the separation to a physical air gap used in plumbing to prevent contaminated water from flowing backward.
Air-gapped systems have been used for military and government networks, industrial-control systems, nuclear-plant controls, aviation equipment, and other environments where compromise could be especially serious. Moving files usually requires removable media, such as a USB drive, or a specially controlled data diode.
An air gap reduces exposure but does not make a system magically invulnerable. A malicious USB device, insider, maintenance connection, or covert channel can bridge the separation. Wireless hardware must also be disabled or removed; a computer unplugged from Ethernet may still have Wi-Fi or Bluetooth. A firewall filters traffic, and a VPN encrypts a connection, but neither physically isolates a system.