An air-gap comes from electronics engineering. An LED was used to transmit a good signal from one circuit to another circuit. This “air gap” allows for blocked other signals from passing from the one circuit to the other.
Air-gapped networks are simply networks with no connection to any other network. They are also known as “disconnected networks”. Unplug the network cable and you have an “air-gapped” network. To get data into an air-gapped network means using a disk or memory stick to transfer data from one outside networks to the air-gapped network. This blocks hacking attempts via the network. More secure air-gapped network still use paper tape to ensure no electronic means exist to infiltrate the air-gapped network.
“still use paper tape”
Very fragile. Yet more backups required.