Which term describes a sudden, unplanned event causing significant damage or loss?

The term that best describes a sudden, unplanned event causing significant damage or loss is "disaster." A disaster typically refers to a catastrophic event that results in widespread disruption, significant harm to people, property, or the environment, and usually requires immediate action and recovery efforts. Disasters can include natural events such as hurricanes or earthquakes, as well as human-made incidents like industrial accidents.

In contrast, an "incident" generally refers to a less severe event that may cause some disruption but does not necessarily lead to significant loss or damage. An "emergency" tends to indicate a situation that requires urgent attention but may not be as extensive or damaging as a disaster. "Risk," on the other hand, relates to the potential for loss or harm but does not describe an event itself. Thus, the most accurate term for a significant, unplanned damaging event is disaster.