The Collaborative International Dictionary of English v.0.48:

Ragnarok \Rag"na*rok"\ (r[.a]"n[.a]*r[o^]k"), Ragnarok
\Rag"na*r["o]k"\ (r[.a]"n[.a]*r[^u]k"), n. [Icel., fr. regin,
   r["o]gn, gods + r["o]k reason, origin, history; confused with
   ragna-r["o]kr the twilight of the gods.] (Norse Myth.)
   The so-called "Twilight of the Gods" (called in German
   G["o]tterd[aum]mmerung), the final destruction of the world
   in the great conflict between the Aesir (gods) on the one
   hand, and on the other, the giants and the powers of Hel
   under the leadership of Loki (who is escaped from bondage).
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Ragnarok
    n 1: myth about the ultimate destruction of the gods in a battle
         with evil [syn: Gotterdammerung, Ragnarok, Twilight of
         the Gods]