An air gapped (aka Air Walled) computer is essentially a technique for keeping a computer or network secure. This is usually done by keeping the computer off the network physically.
This can be done to keep sensitive data on the Air gapped computer or network from being compromised, but can also be done to keep another network safe from an unknown network.
Examples from wikipedia follow:
- Military/governmental computer networks/systems;
- Financial computer systems, such as stock exchanges;
- Industrial control systems, such as SCADA in Oil & Gas fields;
- Life-critical systems, such as: Controls of nuclear power plants;
- Computers used in aviation, such as FADECs and avionics;
- Computerized medical equipment;
- Very simple systems, where there is no need to compromise security in the first place, such as: The engine control unit in an automobile;
- A digital thermostat for temperature and compressor regulation in home HVAC and refrigeration systems;
- Electronic sprinkler controls for watering of lawns.
This Air gapping was discussed on the HBO series The Newsroom during the first episode of the third season, called Boston, where some whistleblower trying to give Neal classified documents asks him to get an “air gapped computer.” In the show they describe this as a computer that is not and has never been connected to a network. While this is not inaccurate, you can see above that there are other scenarios that qualify.