[1] : The Collaborative International Dictionary of English v.0.48

doomsday \dooms"day`\, n. as. d?mes d=ag. see doom, and
   day.
   1. a day of sentence or condemnation; day of death. "my
      body's doomsday." --shak.
      1913 webster

   2. the day of the final judgment.
      1913 webster

            i could not tell till doomsday.       --chaucer.
      1913 webster

   doomsday book. see domesday book.
      1913 webster
see also:
doom day doomsday book domesday book 

[2] : WordNet (r) 2.0

doomsday
     n 1: new testament day of the last judgment when god will
          decree the fates of all men according to the good and
          evil of their earthly lives syn: judgment day, judgement
          day, day of judgment, day of judgement, last
          judgment, last judgement, last day, day of
          reckoning, 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, day of reckoning, end of the world
see also:
judgment day judgement day day of judgment day of judgement last judgment last judgement 
last day day of reckoning crack of doom end of the world doom 
day of reckoning 

Doomsday >> D Words
Doomsday, definition of term: Doomsday
doomsday_pag1.html

Doomsday Book >> D Words
Doomsday Book, definition of term: Doomsday Book
doomsday+book_pag1.html