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.

For vision tasks the threshold of real-time can be seen as the video frame rate, typically 15-30 frames per second. Thus, the total processing time per frame must be below about 0.04 seconds.

Until recently, real-time systems required expensive and dedicated hardware, however recent advances in computer speeds is making this unnecessary, and real time systems are becoming common.