From Longman Dictionary of Contemporary Englishroundelround‧el /ˈraʊndəl/ noun [countable] a circular design, for example one used to make a military aircraft recognizable
Examples from the Corpus
roundel• The main themes were decorated pillars, trefoils and roundels.• Indeed, it shares with it some elements of decoration, e.g. a band of wavecrest pattern around the central roundel.• These figures are in the central roundel, while active seabeasts occupy the semi-roundels.• Number three has a quite different style of roundel and, to me it looks as if the tail stripe colours are reversed!• Number five appears to have a very strange roundel. 