Definition of "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

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

c. 1595–1596 (date written), William Shakespeare, “A Midsommer Nights Dreame”, in Mr. William Shakespeares Comedies, Histories, & Tragedies: Published According to the True Originall Copies (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, [Act II, scene ii], line 1

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