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1530s, "reversal of what is expected" (especially a fatal turning point in a drama), from Latin catastropha, from Greek katastrophe "an overturning; a sudden end," from katastrephein "to overturn, turn down, trample on; to come to an end," from kata "down" (see cata-) + strephein "turn" (see strophe). Extension to "sudden disaster" is first recorded 1748.
in literature, the final action that completes the unraveling of the plot in a play, especially in a tragedy. Catastrophe is a synonym of denouement. The term is sometimes applied to a similar action in a novel or story.