What does real-time mean?

Real-time means computing with a deadline. In a real-time embedded system, the time it takes to respond to an input is a critical component of system performance. If the system produces a correct response, but the response is not generated within the required time limit, the effect may range from a mild nuisance to a catastrophic impact for a safety-related system.

The response of a real-time embedded system to inputs must be both correct and timely. Most standard software development approaches focus on the correctness of the response produced by a piece of code rather than being overly concerned with the timeliness of the response. Non-real-time software development approaches attempt to develop code that executes as quickly as possible, but usually do not provide a hard time limit specifying when the response must be provided. Real-time systems are considered to have failed if the timing constraints are not met, even during stressing and rare combinations...