Wikipedia: An air gap, air wall or air gapping is a network security measure employed on one or more computers to ensure that a secure computer network is physically isolated from unsecured networks, such as the public internet or an unsecured local area network. The name arises from the technique of creating a network that is physically separated (with a conceptual air gap) from all other networks. The air gap may not be completely literal, as networks employing the use of dedicated cryptographic devices that can tunnel packets over untrusted networks while avoiding packet rate or size variation can be considered air gapped, as there is no ability for computers on opposite sides of the gap to communicate.
WhatIs.com: Air gapping is a security measure that involves isolating a computer or network and preventing it from establishing an external connection. For example, an air gapped computer is one that is physically segregated and incapable of connecting wirelessly or physically with other computers or network devices.
Computer Security Resource Center: An interface between two systems at which (a) they are not connected physically and (b) any logical connection is not automated (i.e., data is transferred through the interface only manually, under human control).