Availability - the ability of an IT service or other configuration item (CI) to perform its agreed function when required.
Availability is determined by:
Reliability - a measure of how long IT service or CI can perform its agreed function without interruption.
Maintainability - a measure of how quickly and effectively an IT service or CI can be restored to normal working after a failure.
Serviceability - the ability of a third party supplier to meet the terms of its contract.
High availability - a charachteristic of the IT service that minimizes or masks the effects of IT component failure to the users of a service.
Fault-tolerance - the ability of an IT service, component or CI to operate correctly after failure of a component part.
Continuous operation - an approach to eliminate planned downtime.
Continuous availability - an approach to achieve 100% availability.