Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Come, now a roundel and a fairy song. --Shak.

   2. Anything having a round form; a round figure; a circle.

            The Spaniards, casting themselves into roundels, . .
            . made a flying march to Calais.      --Bacon.
      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.

Source : WordNet®

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: a charge in the shape of a circle; a hollow roundel [syn: {annulet}]