An
air-gapped network is completely isolated from public networks. Data transfer requires physical media (USB, optical) or one-way data diodes. Used for classified systems and critical ICS/SCADA.
Air gaps are not perfect — Stuxnet crossed an air gap via USB. Physical security is critical (USB drops). Data diodes enforce one-way data flow at the hardware level. Consider also electromagnetic isolation (Faraday cage) for classified work.