Air Gap
A security measure that physically isolates a computer or network from the internet and other unsecured networks.
An air gap↗ is a security measure that physically isolates a computer or network from the internet and any other unsecured network, with no wired or wireless connection at all. Data can only move in or out via physical media, like a USB drive, that's manually carried and checked.
Air-gapping is used to protect extremely sensitive systems, classified government networks, nuclear facility controls, and critical financial infrastructure, where the risk of remote compromise must be eliminated entirely rather than merely reduced. It's not foolproof, famous attacks have used infected USB drives to jump the gap, but it remains one of the strongest isolation techniques available.