Real-Time (System) - a system driven by external events (typically sensors that provide input through ADCs) for which services provide computation to produce a response (typically actuators that are interfaced to DACs) before a deadline relative to the event-driven request for service - a hard real-time system must never miss a deadline, but a soft or best effort system may.
Real-Time Clock - a hardware clock circuit which maintains an absolute date and time (e.g. Gregorian or Julian date), often employing a battery-backed clock circuit and/or a method to synchronize with an external time source such as Universal Coordinated Time.
Real-Time Correctness - a real-time service must produce functionally correct outputs and also provide the outputs prior to a relative deadline to be real-time correct.
Re-boot - when a system is commanded or as a part of a recovery mode re-enters the boot code entry point causing re-initialization of memory, I/O interfaces, and re-start of all services.