The Collaborative International Dictionary of English v.0.48:

Doomsday \Dooms"day`\, n. [AS. d?mes d[=a]g. See Doom, and
   Day.]
   1. A day of sentence or condemnation; day of death. "My
      body's doomsday." --Shak.
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   2. The day of the final judgment.
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            I could not tell till doomsday.       --Chaucer.
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   Doomsday Book. See Domesday Book.
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WordNet (r) 3.0 (2006):

Doomsday
    n 1: (New Testament) day at the end of time following Armageddon
         when God will decree the fates of all individual humans
         according to the good and evil of their earthly lives [syn:
         Judgment Day, Judgement Day, Day of Judgment, Day of
         Judgement, Doomsday, Last Judgment, Last Judgement,
         Last Day, eschaton, day of reckoning, doomsday,
         crack of doom, end of the world]
    2: an unpleasant or disastrous destiny; "everyone was aware of
       the approaching doom but was helpless to avoid it"; "that's
       unfortunate but it isn't the end of the world" [syn: doom,
       doomsday, day of reckoning, end of the world]