An air gap is a network-security measure that isolates a computer or network from unsecured networks.
The strongest form of an air gap is physical: the protected system has no wired or wireless connection to outside networks. Organizations may use air-gapped systems for sensitive industrial controls, classified information, cryptocurrency key storage, or backup copies. Some systems described as air-gapped are only logically separated, so their protection depends on configuration rather than complete physical isolation.
Air gaps reduce remote attack paths, but they do not make systems invulnerable. Malware can cross the boundary through removable media, infected maintenance equipment, supply-chain tampering, or human error. Researchers have also demonstrated side-channel techniques that transmit data using sound, light, radio emissions, or other signals.
An air gap is different from a firewall. A firewall filters traffic between connected networks, while an air gap aims to prevent that connection from existing at all.