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An operation within a larger dynamic system is called a real-time operation if the combined reaction- and operation-time of a task is shorter than the maximum delay that is allowed, in view of circumstances outside the operation. The task must also occur before the system to be controlled becomes unstable. A real-time operation is not necessarily fast, as slow systems can allow slow real-time operations. This applies for all types of dynamically changing systems. The polar opposite of a real-time operation is a batch job with interactive timesharing falling somewhere in-between the two extremes.
A typical example could be a computer-controlled braking system in a car. If the driver can stop a car before it hits a wall, the operation was in real-time; if the car hits the wall it was not. Many machines require real-time controllers to avoid "instability", which could lead to the accidental damage or destruction of the system, people, or objects.
Some real-time systems do not have such a constraint on delay as long as input data can be processed rapidly enough so that no backlog occurs. In a real-time Digital signal processing system, the analyzed (input) and/or generated (output) samples (whether they are grouped together in large segments or processed individually) can be processed (or generated) continuously in the time it takes to input and/or output the same set of samples independent of the processing delay. Consider an audio DSP example: if a process requires 2.01 seconds to analyze or process 2.00 seconds of sound, it is not real-time. If it takes 1.99 seconds, it is (or can be made into) a real-time DSP process.
A common life example is that of standing in a line (or queue) waiting for the checkout in a grocery store. If the line asymtotically grows longer and longer without bound, the checkout process is not real-time. If the length of the line is bounded, customers are being "processed" and outputted as rapidly, on average, as they are being inputted, then that process is real-time. The grocer might go out of business or must at least lose business if he/she cannot make his/her checkout process real-time (so it's fundamentally important that this process be real-time).
In the economy real-time-systems are information technologies, which provide real-time access to information or data. The ability of a company to process its data in real-time increases the competitiveness of the company. Real-time processing systems are new technologies and will improve during the next decades. Gartner forcasts a fast increase and use of these real-time systems.
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