roundel

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.”

2. A roundelay or rondelay.

   “Come, now a roundel and a fairy song ... Fairies sing.”

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.”

4. A circular spot; a charge in the form of a small coloured circle.
5. A circular insignia painted on an aircraft to identify its nationality or service.
6. A bastion of a circular form.
7. (India, obsolete)An umbrella.

   “[…] a Roundel-boy; whose business, is, to walk by his master, and defend him with his Roundel or Umbrella from the heat of the sun.”