Air-gapped

Air-gapping is the ultimate security measure: physically isolating a computer or network from the internet and all unsecured connections to make remote hacking impossible.

When compromise is not an option, you cut the cord. Air-gapping physically isolates a secure network or device from the public internet, local area networks, and wireless signals (including Wi-Fi and Bluetooth). It is the gold standard for protecting high-stakes environments: military command systems, nuclear power plants, and critical financial ledgers. Because there is no digital pathway into the system, remote hackers are locked out. To move data, operators must physically transfer files via encrypted removable media or unidirectional data diodes, shifting the security battleground from digital firewalls to strict physical access control.

https://www.ibm.com/topics/air-gap