The Collaborative International Dictionary of English v.0.48:

Roundel \Roun"del\, n. [OF. rondel a roundelay, F. rondel,
   rondeau, a dim. fr. rond; for sense 2, cf. F. rondelle a
   round, a round shield. See Round, a., and cf. Rondel,
   Rondelay.]
   1. (Mus.) A rondelay. "Sung all the roundel lustily."
      --Chaucer.
      [1913 Webster]

            Come, now a roundel and a fairy song. --Shak.
      [1913 Webster]

   2. Anything having a round form; a round figure; a circle.
      [1913 Webster]

            The Spaniards, casting themselves into roundels, . .
            . made a flying march to Calais.      --Bacon.
      [1913 Webster] Specifically:
      (a) A small circular shield, sometimes not more than a
          foot in diameter, used by soldiers in the fourteenth
          and fifteenth centuries.
      (b) (Her.) A circular spot; a sharge in the form of a
          small circle.
      (c) (Fort.) A bastion of a circular form.
          [1913 Webster]

WordNet (r) 3.0 (2006):

roundel
    n 1: English form of rondeau having three triplets with a
         refrain after the first and third
    2: round piece of armor plate that protects the armpit
    3: (heraldry) a charge in the shape of a circle; "a hollow
       roundel" [syn: annulet, roundel]

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "roundel":
   English sonnet, Horatian ode, Italian sonnet, O, Petrarchan sonnet,
   Pindaric ode, Sapphic ode, Shakespearean sonnet, alba, anacreontic,
   annular muscle, annulus, areola, aureole, balada, ballad, ballade,
   bucolic, canso, chanson, chaplet, circle, circuit, circumference,
   circus, clerihew, closed circle, corona, coronet, crown, cycle,
   diadem, dirge, discus, disk, dithyramb, eclogue, elegy, epic,
   epigram, epithalamium, epode, epopee, epopoeia, epos,
   eternal return, fairy ring, garland, georgic, ghazel, glory, haiku,
   halo, idyll, jingle, lasso, limerick, logical circle, loop,
   looplet, lyric, madrigal, magic circle, monody, narrative poem,
   noose, nursery rhyme, ode, orbit, palinode, pastoral,
   pastoral elegy, pastorela, pastourelle, poem, prothalamium, radius,
   rhyme, ring, rondeau, rondel, rondelle, round, roundelay, satire,
   saucer, sestina, sloka, song, sonnet, sonnet sequence, sphincter,
   tanka, tenso, tenzone, threnody, triolet, troubadour poem, verse,
   verselet, versicle, vicious circle, villanelle, virelay, wheel,
   wreath