Any system in which the time at which output is produced is significant. This is usually because the input corresponds to some movement in the physical world, and the output has to relate to that same movement. The lag from input time to output time must be sufficiently small for acceptable timeliness. Timeliness is a function of the total system: missile guidance requires output within a few milliseconds of input whereas scheduling of steamships requires responses measured in days. Real-time systems are usually considered to be those in which the response time is of order milliseconds; interactive systems are those with response times of order seconds and batch systems are those with response times of hours or days. Real-time systems may be subdivided into hard and soft, depending on the severity of failure to meet a deadline for output.

Examples of real-time systems include process control, embedded computer systems, point-of-sale systems, and computer-aided testing.