Air gap

An air gap is a security measure that physically isolates a network (typically OT) from other networks, including the internet, so there is no direct electronic connection between them. Air-gapping was long considered a primary defense for critical infrastructure control systems, on the theory that a system with no network path in can't be remotely attacked.

In practice, true air gaps are increasingly rare: maintenance laptops, USB drives, remote-monitoring tools, and IT/OT convergence all create indirect paths between supposedly air-gapped systems and the outside world.