Airside is the other side of the burly chap asking for your airside Pass, who will also require you to go through a scanner as well as anything you are carrying.
That's in a building where there is a landside and an airside; terminal, obviously, but also hangars, ATC facilities, sometimes even the RFFS building has a landside area. There are others, but I can't think of them.
Outside, the manoeuvering area (runway, aprons, parking areas, taxiways etc) is all airside, the Customs area is all airside, anywhere inside the security fence (aka boundary, probably) is airside. Public access areas are landside.
The airside/landside barrier is defined and drawn at any airport for every single metre of its route around the airside. It is management's job to make sure that it is impenetrable except through manned access points. That means any gap, say between buildings, of even a few centimetres must be blocked up. It is a criminal offence to enter the airside of an airport without permission of the airport authority; this is one reason why it is so carefully defined. The other is Security, of course; the heavies need to know when you cross the dead-line, otherwise they might shoot you by mistake.