roundel

Etymology

From Middle English roundel, rundel, rondel, from Old French rondel (“something round and flat”), a diminutive of rond (“round”). More at round.

noun

1. Anything having a round form; a round figure; a circle.
   the London Underground roundel
   The Overground has been given a seat at the Underground table. It has the roundel, the Johnston typeface, and it is on the Tube map. 2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, page 93
2. (music) A roundelay or rondelay.
3. A small circular shield, sometimes not more than a foot in diameter, used by soldiers in the fourteenth and fifteenth centuries.
   The roundel or rondache derived its name from its circular figure, it was made of oziers boards of light wood, sinews or ropes, covered with leather, plates of metal, or stuck full of nails in concentric circles or other figures. 1786, Francis Grose, A Treatise on Ancient Armour and Weapons, page 26
4. (heraldry) A circular spot; a charge in the form of a small coloured circle.
5. (aviation) A circular insignia painted on an aircraft to identify its nationality or service.
6. A bastion of a circular form.