catastrophe

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A great, often sudden calamity.
2. n. A complete failure; a fiasco: The food was cold, the guests quarreled—the whole dinner was a catastrophe.
3. n. The concluding action of a drama, especially a classical tragedy, following the climax and containing a resolution of the plot.
4. n. A sudden violent change in the earth's surface; a cataclysm.

Century Dictionary and Cyclopedia

1. n. The arrangement of actions or interconnection of causes which constitutes the final event of a dramatic piece; the unfolding and winding up of the plot, clearing up difficulties, and closing the play; the dénouement. The ancients divided a play into the protasis, epitasis, catastasis, and catastrophe; that is, the introduction, continuance, heightening, and development or conclusion.
2. n. A notable event terminating a connected series; a finishing stroke or wind-up; specifically, an unfortunate conclusion; hence, any great calamity or disaster, especially one happening suddenly or from an irresistible cause.
3. n. In geology, an occurrence of geological importance not in harmony with preceding events, and not the result of causes acting always in a given direction; a cataclysm. It was once generally believed that the earth has “undergone a succession of revolutions and aqueous catastrophes interrupted by long intervals of tranquillity” (Lyell). The deluge was one of these great catastrophes. A similar view is the once common idea that all the living organisms on the earth's surface had been again and again exterminated, to be succeeded by new creations of plants and animals.
4. n. Synonyms Disaster, Calamity, etc. (see mis-fortune); consummation, finale.

Wiktionary

1. n. Any large and disastrous event of great significance.
2. n. insurance A disaster beyond expectations
3. n. narratology The dramatic event that initiates the resolution of the plot in a tragedy.
4. n. mathematics A type of bifurcation, where a system shifts between two stable states.

GNU Webster's 1913

1. n. An event producing a subversion of the order or system of things; a final event, usually of a calamitous or disastrous nature; hence, sudden calamity; great misfortune.
2. n. The final event in a romance or a dramatic piece; a denouement, as a death in a tragedy, or a marriage in a comedy.
3. n. (Geol.) A violent and widely extended change in the surface of the earth, .

WordNet 3.0

1. n. an event resulting in great loss and misfortune
2. n. a state of extreme (usually irremediable) ruin and misfortune
3. n. a sudden violent change in the earth's surface