a state of extreme (usually irremediable) ruin and misfortune
"lack of funds has resulted in a catastrophe for our school system"
"his policies were a disaster"
an event resulting in great loss and misfortune
"the whole city was affected by the irremediable calamity"
"the earthquake was a disaster"
an act that has disastrous consequences
"The CEO called the company's decision to ignore safety regulations a disaster after three workers were injured."
An unexpected natural or man-made catastrophe of substantial extent causing significant physical damage or destruction, loss of life or sometimes permanent change to the natural environment.
"The sudden earthquake leveled thousands of homes and altered the coastline permanently, marking one of the worst disasters in the region's history."
In plain English: A disaster is a sudden and terrible event that causes great damage or loss.
"The heavy rain turned our picnic into a total disaster."
Usage: Use "disaster" to describe a sudden event that causes severe damage, injury, or death, such as a hurricane or a major industrial accident. Avoid using it casually for minor setbacks or ordinary bad luck.
The word disaster comes from the Italian phrase meaning "bad star," which was borrowed into Middle French and then English to describe an event believed to be caused by unfavorable celestial alignment. Although its roots trace back through Latin and Ancient Greek, the term originally referred specifically to a misfortune linked to planetary positions before taking on its modern sense of any catastrophic occurrence.