As fast as required. A real-time system must respond to a signal, event or request fast enough to satisfy some requirement. Real time often refers to process control and embedded systems. For example, space flight computers must respond to changing conditions in order to keep the rocket ship on course. Industrial robots must respond quickly in order to keep the assembly line moving at full speed. Antilock brakes and other driving assist systems must respond to changing road conditions immediately in order to benefit the driver and vehicle. See embedded system, real-time audio and real-time video.

Videoconferencing and Gaming
Videoconferencing requires real-time systems that can transmit video across the network and display it at the other end as quickly as the camera is recording it. Gaming is another real-time application. The system must respond to the user's inputs and render video frames on screen fast enough to simulate realistic motion.

Business Systems - Not Really
The typical transaction processing system is not considered real time even though it does update databases immediately. However, in the past, as business information systems became fast enough to respond to queries and updates in only a second or two, they were called real-time systems. See online.