Reliability, availability and serviceability

Definition
Reliability, availability, and serviceability (RAS) is a computer hardware engineering term. The phrase was originally used by IBM as a term to describe the robustness of their mainframe computers.

Computers designed with higher levels of RAS have a multitude of features that protect data integrity and help them stay available for long periods of time without failure — this data integrity and uptime is a particular selling point for mainframes and fault-tolerant systems.