A small circular shield, a target. Compare roundel. Now historical.
The term is principally used in early use to describe French armoury; later antiquaries and collectors have applied the term (without historical justification) to any large round shield, and particularly to large iron parade-shields decorated with embossed scenes popular during the 16th and 17th centuries..
Origin
Late 16th century. From Middle French, French rondache, †rondace circular shield, foot soldier armed with such a shield, probably alteration (with substitution of -ache, suffix forming nouns usually with pejorative connotation) of rondelle. In some forms probably after Dutch rondas.