The Collaborative International Dictionary of English v.0.48:

Dithyramb \Dith"y*ramb\, n. [L. dithyrambus, Gr. ? a kind of
   lyric poetry in honor of Bacchus; also, a name of Bacchus; of
   unknown origin: cf. F. dithyrambe.]
   A kind of lyric poetry in honor of Bacchus, usually sung by a
   band of revelers to a flute accompaniment; hence, in general,
   a poem written in a wild irregular strain. --Bentley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dithyramb
    n 1: a wildly enthusiastic speech or piece of writing
    2: (ancient Greece) a passionate hymn (usually in honor of
       Dionysus)