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]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "cataclysm":
   accident, alluvion, alluvium, apoplexy, bloodless revolution, blow,
   bouleversement, breakdown, breaking up, breakup, calamity,
   casualty, cataract, catastrophe, cave, cave-in, clean slate,
   clean sweep, climax, collapse, collision, computer revolution,
   contretemps, convulsion, counterrevolution, crack-up, crash,
   debacle, deluge, diastrophism, disaster, engulfment, fit, flood,
   flooding, grief, ill hap, inundation, misadventure, mischance,
   misfortune, mishap, nasty blow, overflow, overflowing, overrunning,
   overthrow, overturn, palace revolution, paroxysm, pileup, pour,
   quake, radical change, revolt, revolution, revolutionary war,
   revulsion, shipwreck, shock, smash, smashup, spasm, spate, spill,
   spillage, staggering blow, striking alteration, stroke, submersion,
   subversion, sweeping change, tabula rasa, technological revolution,
   temblor, the Deluge, the Flood, tidal wave, torrent, total change,
   total loss, tragedy, transilience, tsunami, upheaval, upset,
   violent change, washout, whelming, wrack, wreck