Source : Webster's Revised Unabridged Dictionary (1913)

Dies Irae \Di"es I"r[ae]\
   Day of wrath; -- the name and beginning of a famous
   medi[ae]val Latin hymn on the Last Judgment.

Source : WordNet®

Dies Irae
     n : the first words of a medieval Latin hymn describing the Last
         Judgment (literally `day of wrath')