roundel

noun7

1. •
   Anything having a round form; a round figure; a circle.

   • the London Underground roundel
2. •
   A circular spot; a charge in the form of a small coloured circle.
3. •
   A circular insignia painted on an aircraft to identify its nationality or service.
4. •
   A roundelay or rondelay.

   • Come, now a roundel and a fairy song ... Fairies sing. — 1595
5. •
   A bastion of a circular form.
6. •
   A small circular shield, sometimes not more than a foot in diameter, used by soldiers in the fourteenth and fifteenth centuries.
7. •
   (India, no longer used) 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. — 1773