The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Geol.) Any violent catastrophe, involving sudden and
      extensive changes of the earth's surface.
      [1913 Webster] Cataclysmal

WordNet (r) 3.0 (2006):

cataclysm
    n 1: a sudden violent change in the earth's surface [syn:
         catastrophe, cataclysm]
    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, cataclysm]