A measure of the ability of a system to recover from error conditions, whether generated externally or internally; for example, a robust system would be tolerant to errors in input data or to failures of internal components. Although there may be a relationship between robustness and reliability, the two are distinct measures: a system never called upon to recover from error conditions may be reliable without being robust; a highly robust system that recovers and continues to operate despite numerous error conditions may still be regarded as unreliable in that it fails to provide essential services in a timely fashion on demand.