Answer Description

This setup is best known as an air gap. In network an air gap means two or more networks are physically separated from each other to ensure no data can traverse from one to the other. Generally if a network is so critical it requires an air gap it will be a completely stand alone network with no access to other networks and especially the internet. A true air gap is not common in most businesses, but some known examples are government or military networks, highly critical infrastructure networks like nuclear power plant controls and financial systems like stock exchanges.

Wikipedia

An air gap, air wall, air gapping or disconnected network is a network security measure employed on one or more computers to ensure that a secure computer network is physically isolated from unsecured networks, such as the public Internet or an unsecured local area network. It means a computer or network has no network interface controllers connected to other networks, with a physical or conceptual air gap, analogous to the air gap used in plumbing to maintain water quality.