Meaning – The term RAS, or Reliability, availability, and serviceability, refers to a computer hardware engineering term involving reliability engineering, high availability, and serviceability design. The phrase was originally used by International Business Machines (IBM) as a term to describe the robustness of their mainframe computers.
Reliability can be defined as the probability that a system will produce correct outputs up to some given time t. Reliability is enhanced by features that help to avoid, detect and repair hardware faults.
Availability means the probability that a system is operational at a given time, i.e. the amount of time a device is actually operating as the percentage of the total time it should be operating.
Serviceability or maintainability is the simplicity and speed with which a system can be repaired or maintained; if the time to repair a failed system increases, then availability will decrease.