We give a formal definition of what it means for a system to "tolerate" a class of "faults". The definition consists of two conditions: One, if a fault occurs when the system state is within a set of "legal" states, the resulting state is within some larger set and, if faults continue occurring, the system state remains within that larger set (Closure). And two, if faults stop occurring, the system eventually reaches a state within the legal set (Convergence). We demonstrate the applicability of our definition for specifying and verifying the fault-tolerance properties of a variety of digital and computer systems. Further, using the definition, we obtain a simple classification of fault-tolerant systems and discuss methods for their systematic design. as traditionally been studied in the context of specifi...