The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Note: The text of the Dies Irae follows:
         DIES IRAE attributed to Tomaso de Celano
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         Latin English
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         Dies irae, dies illa The day of wrath, that day solvet
         saeclum in favilla: which will reduce the world to
         ashes, teste David cum Sibylla. as foretold by David
         and the Sybil. Quantus tremor est futurus, What terror
         there will be, quando judex est venturus, when the Lord
         will come cuncta stricte discussurus! to rigorously
         judge all! Tuba mirum spargens sonum The trumpet,
         scattering a wondrous sound per sepulcra regionum,
         among the graves of all the lands, coget omnes ante
         thronum. will assemble all before the Throne. Mors
         stupebit et natura, Death and Nature will be astounded
         cum resurget creatura, when they see a creature rise
         again judicanti responsura. to answer to the Judge.
         Liber scriptus proferetur, The book will be brought
         forth in quo totum continetur, in which all deeds are
         noted, unde mundus judicetur. for which humanity will
         answer. Judex ergo cum sedebit, When the judge will be
         seated, quidquid latet apparebit, all that is hidden
         will appear, nil inultum remanebit. and nothing will go
         unpunished. Quid sum miser tunc dicturus? Alas, what
         will I then say? Quem patronum rogaturus, To what
         advocate shall I appeal, cum vix justus sit securus?
         when even the just tremble? Rex tremendae majestatis, O
         king of redoutable majesty, qui salvandos salvas
         gratis, who freely saves the elect, salva me fons
         pietatis. save me, o fount of piety! Recordare Jesu
         pie, Remember, good Jesus, quod sum causa tuae viae,
         that I am the cause of your journey, ne me perdas illa
         die. do not lose me on that day. Quaerens me sedisti
         lassus, You wearied yourself in finding me. redemisti
         crucem passus, You have redeemed me through the cross.
         tantus labor non sit cassus. Let not such great efforts
         be in vain. Juste judex ultionis, O judge of vengeance,
         justly donum fac remissionis make a gift of your
         forgiveness ante diem rationis. before the day of
         reckoning. Ingemisco tanquam reus, I lament like a
         guilty one. culpa rubet vultus meus, My faults cause me
         to blush, supplicanti parce, Deus. I beg you, spare me.
         Qui Mariam absolvisti, You who have absolved Mary et
         latronem exaudisti, and have heard the thief's prayer,
         mihi quoque spem dedisti. have also given me hope.
         Preces meae non sunt dignae: My prayers are not worthy,
         sed tu bonus fac benigne, but you, o Good One, please
         grant freely ne perenni cremer igne. that I do not burn
         in the eternal fire. Inter oves locum praesta, Give me
         a place among the sheep, et ab haedis me sequestra,
         separate me from the goats statuens in parte dextra. by
         placing me at your right. Confutatis maledictis, Having
         destroyed the accursed, flammis acribus addictis:
         condemned them to the fierce flames, voca me cum
         benedictis. Count me among the blessed. Oro Supplex Et
         Acclinis, I prostrate myself, supplicating, Cor
         Contritum Quasi Cinis, my heart in ashes, repentant;
         Gere Curam Mei Finis. take good care of my last moment!
         Lacrimosa dies illa, That tearful day, qua resurget ex
         favilla when shall rise again from the ashes judicandus
         homo reus. sinful man to be judged. Huic ergo parce,
         Deus: Therefore pardon him, o God. pie Jesu Domine,
         Holy Lord Jesus, dona eis requiem. give them rest.
         Amen. Amen.
         [PJC]

WordNet (r) 3.0 (2006):

Dies Irae
    n 1: the first words of a medieval Latin hymn describing the
         Last Judgment (literally `day of wrath')