Source : Webster's Revised Unabridged Dictionary (1913)

Cataclysm \Cat"a*clysm\, n. [L. cataclysmos, Gr. kataklysmo`s,
   from ? to dash over, inundate; kata` downward, against + ? to
   wash or dash over: cf. F. cataclysme.]
   1. An extensive overflow or sweeping flood of water; a
      deluge.

   2. (Geol.) Any violent catastrophe, involving sudden and
      extensive changes of the earth's surface.

Source : WordNet®

cataclysm
     n 1: a sudden violent change in the earth's surface [syn: {catastrophe}]
     2: an event resulting in great loss and misfortune; "the whole
        city was affected by the irremediable calamity"; "the
        earthquake was a disaster" [syn: {calamity}, {catastrophe},
         {disaster}, {tragedy}]