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]