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The capability of the software product to maintain a specified level of performance in cases of software faults or of infringement of its specified interface. [ ISO/IEC 9126-1 ]

Other Definitions

Reliability [ ISO/IEC/IEEE 24765
The ability of a system or component to perform its required functions under stated conditions for a specified period of time.
Software Reliability [ ISO/IEC/IEEE 24765
The probability[1] that software will not cause the failure of a system for a specified time under specified conditions.


  • ISO/IEC 9126-1 provides the exact same definition for reliability and Fault Tolerance.
  • Wear or ageing does not occur in software. Limitations in reliability are due to faults in requirements, design, and implementation. Failures due to these faults depend on the way the software product is used and the program options selected rather than on elapsed time. [ ISO/IEC 9126-1 ]
  • The definition of reliability in ISO/IEC 2382-14:1997 is "The ability of functional unit to perform a required function...". In ISO/IEC 9126-1, functionality is only one of the characteristics of software quality. Therefore, the definition of reliability has been broadened to "maintain a specified level of performance..." instead of "...perform a required function". [ ISO/IEC 9126-1 ]

See also




  1. The probability is a function of the inputs to and use of the system as well as a function of the existence of faults in the software. The inputs to the system determine whether existing faults, if any, are encountered.
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