The Collaborative International Dictionary of English v.0.48:

Rondel \Ron"del\, n. [Cf. Rondeau, Roundel.]
   1. (Fort.) A small round tower erected at the foot of a
      bastion. [Obs.]
      [1913 Webster]

   2. [F.]
      (a) Same as Rondeau.
      (b) Specifically, a particular form of rondeau containing
          fourteen lines in two rhymes, the refrain being a
          repetition of the first and second lines as the
          seventh and eighth, and again as the thirteenth and
          fourteenth. --E. W. Gosse.
          [1913 Webster]

WordNet (r) 3.0 (2006):

rondel
    n 1: 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]