An unplanned event that causes the general system or major application to be inoperable for an unacceptable length of time (e.g., minor or extended power outage, extended unavailable network, or equipment or facility damage or destruction).
Sources:
CNSSI 4009-2015 from NIST SP 800-34 Rev. 1 - Adapted

  An unplanned event that causes an information system to be inoperable for a length of time (e.g., minor or extended power outage, extended unavailable network, or equipment or facility damage or destruction).
Sources:
NIST SP 800-34 Rev. 1 under Disruption