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.
      [1913 Webster]

   Note: The word calamity was first derived from calamus when
         the corn could not get out of the stalk. --Bacon.
         [1913 Webster]

               Strokes of calamity that scathe and scorch the
               soul.                              --W. Irving.
         [1913 Webster]

   2. A state or time of distress or misfortune; misery.
      [1913 Webster]

            The deliberations of calamity are rarely wise.
                                                  --Burke.
      [1913 Webster]

            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.
          [1913 Webster]

WordNet (r) 3.0 (2006):

calamity
    n 1: 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:

68 Moby Thesaurus words for "calamity":
   accident, adversity, affliction, bane, blow, bugbear, burden,
   casualty, cataclysm, catastrophe, collapse, collision, contretemps,
   crack-up, crash, cross, crushing burden, curse, death, desolation,
   destruction, devastation, disaster, disease, distress, evil,
   fatal accident, fatality, grief, grievance, hardship, harm,
   ill hap, infliction, misadventure, mischance, misery, misfortune,
   mishap, nasty blow, nemesis, open wound, pest, pestilence, pileup,
   plague, reverse, ruin, ruination, running sore, scourge, shipwreck,
   shock, smash, smashup, staggering blow, thorn, torment, tragedy,
   trial, tribulation, trouble, vexation, violent death, visitation,
   woe, wreck, wretchedness



The Devil's Dictionary (1881-1906):

CALAMITY, n.  A more than commonly plain and unmistakable reminder
that the affairs of this life are not of our own ordering.  Calamities
are of two kinds:  misfortune to ourselves, and good fortune to
others.