Learn in the textbookReliability Design
DefinitionFault tolerant (フォールトトレラント) design arranges a system so that, even if part of it fails, the whole keeps processing.
ExplanationIt starts from the premise that failures can never be eliminated and builds in advance a configuration that copes when something breaks. A redundant configuration can be seen as this idea put into practice at the level of whole systems. The noun form is fault tolerance (フォールトトレランス).
AnalogyAn airliner with several engines can keep flying on the others if one of them quits.
Memory hookFault means a failure and tolerant means able to bear it: a system that bears its faults and keeps going.