[Jtsummers]

It is pretty much the definition for real-time for anyone who has worked on embedded, safety, and critical systems. Preemption is about priority, which is a property of parts of the real-time system. A real-time system with only one kind of thing it's doing doesn't have to deal with priority (too much), but in communication systems (like between aircraft and air traffic control) there may be messages which will always be processed within some unit of time, and others which can be deferred or even discarded if they are preempted by a higher priority message. The whole system is still real-time. Usually with a fixed processing cycle. For a radio, maybe every 1/100th of a second it will receive, process, and transmit, repeat. Priorities determine what gets transmitted and processed in any given cycle based on various inputs from the aircraft, the aircraft operator, or other radios in the network.

That 1/100th of a second cycle time and the guarantees associated with it are what make it a real-time system (a hard real-time system, in this example).