The Collaborative International Dictionary of English v.0.48:

Rondeau \Ron*deau"\, n. [F. See Roundel.] [Written also
   rondo.]
   1. A species of lyric poetry so composed as to contain a
      refrain or repetition which recurs according to a fixed
      law, and a limited number of rhymes recurring also by
      rule.
      [1913 Webster]

   Note: When the rondeau was called the rondel it was mostly
         written in fourteen octosyllabic lines of two rhymes,
         as in the rondels of Charles d'Orleans. . . . In the
         17th century the approved form of the rondeau was a
         structure of thirteen verses with a refrain. --Encyc.
         Brit.
         [1913 Webster]

   2. (Mus.) See Rondo, 1.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rondeau
    n 1: a musical form that is often the last movement of a sonata
         [syn: rondo, rondeau]
    2: a French verse form of 10 or 13 lines running on two rhymes;
       the opening phrase is repeated as the refrain of the second
       and third stanzas [syn: rondeau, rondel]

Moby Thesaurus II by Grady Ward, 1.0:

77 Moby Thesaurus words for "rondeau":
   English sonnet, Horatian ode, Italian sonnet, Petrarchan sonnet,
   Pindaric ode, Sapphic ode, Shakespearean sonnet, alba, anacreontic,
   balada, ballad, ballade, bucolic, canon, canso, catch, chanson,
   clerihew, dirge, dithyramb, eclogue, elegy, epic, epigram,
   epithalamium, epode, epopee, epopoeia, epos, fugato, fugue,
   georgic, ghazel, haiku, idyll, jingle, limerick, lyric, madrigal,
   monody, narrative poem, nursery rhyme, ode, palinode, pastoral,
   pastoral elegy, pastorela, pastourelle, poem, prothalamium, rhyme,
   rondel, rondelet, rondino, rondo, rondoletto, round, roundel,
   roundelay, satire, sestina, sloka, song, sonnet, sonnet sequence,
   tanka, tenso, tenzone, threnody, triolet, troll, troubadour poem,
   verse, verselet, versicle, villanelle, virelay