a picture or pattern contained in a circle:the calendar is painted with roundels of the Labours of the Months
Heraldry a plain filled circle as a charge (often with a special name according to colour).
a circular identifying mark painted on military aircraft, as, for example, the red, white, and blue of the RAF.
2a short poem consisting of three stanzas of three lines each, rhyming alternately, with the opening words repeated as a refrain after the first and third stanzas. The form, a variant of the rondeau, was developed by Swinburne.
Middle English: from Old French rondel, from ro(u)nd-(see round)
roundel in other Oxford dictionaries
Definition of roundel in the US English dictionary