You can have an air gap between two physical items - it doesn't matter if those physical items are air tight or not. Air gapped doesn't mean the items are prohibited to intake air (i.e. air tight), it just means they're prohibited to intake things _apart_ from air.
Historically, we did not have wifi and other radio based new fangled data communications. Data connectivity required wires, physical connections. If there was a gap between the two devices that had no wire, just air, that was air gapped. No comms could happen between the two. It is physically isolated. it used to be called "physically isolated" when we used it in the 80's (?). Some say, we stole it from plumbers but that is hogwash (pun intended, you know the backflow prevention thing). I vaguely recall start seeing it late 1990's to 2K in the public?
Mission Impossible 1996 the computer in the room where tom cruise is lowered into the room. That was an example of 90's air-gapped system.
The name stuck because it sounds cool. In my opinion, there is no such thing as true "air-gapped network" any more. There are too many ways to snoop on systems that are isolated, without "physical" and radio connections in the traditional sense (e.g., listen to the "electricity", sounds, power fluctuation, ground vibration, squirrel squeeks).
Airgapped systems have an air gap between the system and the wider world. The only way to move data to and from them is for someone to walk across the gap with physical media.
There are no communication cables between the host system and the wider world.
* air-gap malware can be designed to communicate secure information acoustically, at frequencies near or beyond the limit of human hearing.
* In 2014, researchers introduced ″AirHopper″, a bifurcated attack pattern showing the feasibility of data exfiltration from an isolated computer to a nearby mobile phone, using FM frequency signals.
* In 2015, "HELLONE", a covert signaling channel between air-gapped computers using thermal manipulations, was introduced. "BitWhisper" supports bidirectional communication and requires no additional dedicated peripheral hardware.
* Later in 2015, researchers introduced "GSMem", a method for exfiltrating data from air-gapped computers over cellular frequencies. The transmission - generated by a standard internal bus - renders the computer into a small cellular transmitter antenna.