In computer security, the practice of physically segregating a portion of the network, leaving it disconnected from other segments of the LAN/WAN and therefore not accessible from the Internet.

Note that using wireless networking to circumvent an air gap is counterproductive; the point is that the private/confidential/secret part of your network is completely disconnected from the public areas of your network.

Air gap. (Physics)

An air-filled gap in a magnetic or electric circuit; specif., in a dynamo or motor, the space between the field-magnet poles and the armature; clearance.

 

© Webster 1913