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]