Roundel

noun

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

   "the London Underground roundel"

2. 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.
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.