RAS – Reliability, Availability, Serviceability. Reliability means the ability of a computer hardware or software item to perform according to its specifications. Availability is the ratio of time a system or component is functional to the total time it is required or expected to function. Serviceability refers to the ease with which a component, device or system can be maintained and repaired. The RAS acronym and term was originally devised by IBM in connection with its mainframe systems.