Exerpts from the Oxford English Dictionary:
(di'za:stǝ(r), ... 'a disaster, misfortune, calamitie, misadventure, hard chance,;... astre 'a starre, a Planet; also destinie, fate, fortune, hap'...
1. A unfavourable aspect of a star or a planet; 'an obnoxious planet'.
2. Anything that befalls of ruinous or distressing nature; a sudden or great misfortune, mishap, or misadventure; a calamity.
A somewhat more contemporary definition from Wikipedia:
A disaster is the tragedy of a natural or man-made hazard (a hazard is a situation which poses a level of threat to life, health, property, or environment) that negatively affects society or environment.
In contemporary academia, disasters are seen as the consequence of inappropriately managed risk. These risks are the product of hazards and vulnerability.
think disaster: a blog by Scott Reuter (@sct_r) 