Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. The day of the final judgment.

            I could not tell till doomsday.       --Chaucer.

   {Doomsday Book}. See {Domesday Book}.

Source : WordNet®

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}]