The Collaborative International Dictionary of English v.0.48:

Nibelungenlied \Ni"be*lung`en*lied`\, n. [G. See Nibelungs;
   Lied.]
   A great medieval German epic of unknown authorship containing
   traditions which refer to the Burgundians at the time of
   Attila (called Etzel in the poem) and mythological elements
   pointing to heathen times.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Nibelungenlied
    n 1: an epic poem written in Middle High German and based on the
         legends of Siegfried and Teutonic kings