What is defined as the ability of a service, component or configuration item (CI) to perform its agreed function when required?

Serviceability refers to a third-party supplier's contractual obligation to maintain or repair a CI, not the ability of the item itself to perform its function.

Availability is formally defined in ITIL as the ability of a service, component, or configuration item to perform its agreed function when required. It is a core metric managed by the Availability Management process. It is typically expressed as a percentage of agreed service time that the service is actually functioning.

Capacity relates to the maximum throughput or volume a service or component can handle, not its ability to perform its agreed function when needed.

Continuity refers to IT Service Continuity Management, which ensures services can be recovered and restored after a disaster or major disruption.