1. [n]  an act that has disastrous consequences
2. [n]  an event resulting in great loss and misfortune; "the whole city was affected by the irremediable calamity"; "the earthquake was a disaster"
3. [n]  a state of extreme (usually irremediable) ruin and misfortune; "lack of funds has resulted in a catastrophe for our school system"; "his policies were a disaster"

Disaster
Volume eight of L. Ron Hubbard`s massive Mission Earth series follows the adventures of Jettero Heller as he tries to thwart the ever more convoluted plots of the alien Voltarian government to destroy the Earth as we know it.

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1. \Dis*as"ter\, n. [F. d['e]sastre; pref. d['e]s- (L.
   dis-) + astre star, fr. L. astrum; a word of astrological
   origin. See {Aster}, {Astral}, {Star}.]
   1. An unpropitious or baleful aspect of a planet or star;
      malevolent influence of a heavenly body; hence, an ill
      portent. [Obs.]
            Disasters in the sun.                 --Shak.
   2. An adverse or unfortunate event, esp. a sudden and
      extraordinary misfortune; a calamity; a serious mishap.
            But noble souls, through dust and heat, Rise from
            disaster and defeat The stronger.     --Longfellow.
   Syn: Calamity; misfortune; mishap; mischance; visitation;
        misadventure; ill luck. See {Calamity}.
   

2. \Dis*as"ter\, v. t.
   1. To blast by the influence of a baleful star. [Obs.] --Sir
      P. Sidney.
   2. To bring harm upon; to injure. [R.] --Thomson.