The BEST description of an incident is:

This choice incorrectly adds the qualifier 'unless there is a backup to that service,' which has no place in the ITIL definition - an incident occurs regardless of whether a backup or workaround is available.

The ITIL definition of an incident is 'an unplanned interruption to an IT service or a reduction in the quality of an IT service.' This definition is precise on two points: the disruption must be unplanned, and it covers both complete outages and degraded service quality, ensuring all service-affecting events are captured and managed through the incident management process.

Incidents are specifically unplanned disruptions; planned events such as scheduled maintenance windows are not classified as incidents under ITIL.

An incident is defined by its impact on service quality or availability, not by whether it has been reported to the service desk - unreported disruptions are still incidents.