Availability relates to information being available when required by the process now and in the future. It also concerns the safeguarding of necessary resources and associated capabilities.
Corresponds to accessibility and security goal.
Source: CobiT 5
Ability of a Configuration Item or IT Service to perform its agreed function when required. Availability is determined by reliability, maintainability, serviceability, performance, and security. Availability is usually calculated as a percentage. This calculation is often based on Agreed Service Time and Downtime.