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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 gets 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 opposite of a real-time operation is a batch job.

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.