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.

Example of usage“Computers designed with higher levels of RAS have many features that protect data integrity and help them stay available for long periods of time without failure.”