Real time is a term that pertains to the responsiveness of a system wich is sufficiently immediate or that enables it to keep up with some external process.

From the FAQ for the comp.realtime newsgroup: ``A real-time system is one in which the correctness of the computations not only depends upon the correctness of the computation but also upon the time at which the result is produced. If the timing constraints of the system are not met, system failure is said to have occurred.''

"real-time" adds a time constraint to the notion of a program being correct. The phrase "real-time" does not directly relate to how fast the program responds, even though many people believe that real-time means real-fast. This is a direct fall-out from the fact that it is easier to meet deadlines with a fast system. However, many operating systems now run on powerful hardware and are "fast", but that speed does not necessarily imply "determinism". Determinism is more important than average speed for real-time systems.

From Burns and Wellings (Burns & Wellings, 2001), ``any information processing activity or system which has to respond to externally generated input stimuli within a finite and specified delay.'' in hard real-time systems, it is ``absolutely imperative that responses occur within the specified deadline'' in soft real-time systems, ``response times are important, but the system will still function correctly if deadlines are occasionally missed.''.

Or you could say it this way: "not only functional but also temporal behaviour are important"

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