"Real-time" seems to be a cool term in conversations about machine vision, but raising the question about the precise meaning may trigger some stimulating discussions. "Real-time systems" often are simply assumed to be "fast", whereas some people use the term for image processing at the frame-rate of a consumer video stream. This article tries to give a definition of real-time processing within the framework of the inspection of moving objects in a production line. The main features are the ability of a real-time system to react to asynchronous sensor signals at any time and to call back within a defined time interval.