Air gap
Physically isolated. No cable, no Wi-Fi, no 'just this jump host' to the internet. Removable media is still a path if you let it be.
An air gap is physical isolation from other networks, especially the internet. The intent is that less-trusted networks cannot talk to the high-value one. A VLAN with a stern name is not this. A cable is this.
People and media still cross the gap. Control removable media, build a real transfer process, and stop pretending a laptop that 'only sometimes' gets email is air-gapped.
Fact source: ASD's ACSC glossary.