The Collaborative International Dictionary of English v.0.48:

Calamity \Ca*lam"i*ty\n.; pl. Calamities. [L. calamitas, akin
   to in-columis unharmed: cf. F. calamit['e]]
   1. Any great misfortune or cause of misery; -- generally
      applied to events or disasters which produce extensive
      evil, either to communities or individuals.
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   Note: The word calamity was first derived from calamus when
         the corn could not get out of the stalk. --Bacon.
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               Strokes of calamity that scathe and scorch the
               soul.                              --W. Irving.
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   2. A state or time of distress or misfortune; misery.
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            The deliberations of calamity are rarely wise.
                                                  --Burke.
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            Where'er I came I brought calamity.   --Tennyson.

   Syn: Disaster; distress; affliction; adversity; misfortune;
        unhappiness; infelicity; mishap; mischance; misery;
        evil; extremity; exigency; downfall.

   Usage: Calamity, Disaster, Misfortune, Mishap,
          Mischance. Of these words, calamity is the
          strongest. It supposes a somewhat continuous state,
          produced not usually by the direct agency of man, but
          by natural causes, such as fire, flood, tempest,
          disease, etc, Disaster denotes literally ill-starred,
          and is some unforeseen and distressing event which
          comes suddenly upon us, as if from hostile planet.
          Misfortune is often due to no specific cause; it is
          simply the bad fortune of an individual; a link in the
          chain of events; an evil independent of his own
          conduct, and not to be charged as a fault. Mischance
          and mishap are misfortunes of a trivial nature,
          occurring usually to individuals. "A calamity is
          either public or private, but more frequently the
          former; a disaster is rather particular than private;
          it affects things rather than persons; journey,
          expedition, and military movements are often attended
          with disasters; misfortunes are usually personal; they
          immediately affect the interests of the individual."
          --Crabb.
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