The probability that an item will perform a required function, under stated conditions, for a stated period of time.
The proportion of time that an item is capable of operating to specification within a large time interval. Highly available system could be expensive to maintain.
The probability that a failed product will be restored to operational effectiveness within a given period of time when the repair action is performed in accordance with prescribed procedures.
The probability of failure-free software operation for a specified period of time in a specified environment. Software failures are systemic failures revealed randomly.
RAM performance of the whole system is computed from its subsystems using RAM modelling. Product reliability does not guarantee high system reliability. System reliability depends on asset population, asset reliability, redundancy, low common cause failures and software reliability.