roundel

A curved form, especially a semicircular panel, window, or recess.

A rondel.

A rondeau.

In the fine arts, a composition or design contained within a circle, a type much favored by the painters and sculptor’s of the quattrocento in Italy: found also in excavations at Cnosus in Crete; also a wooden platter painted. See tondo.

Anything round; a round form or figure; a circle, or something of circular form.

Specifically— In heraldry, a circular figure used as a bearing, and commonly blazoned, not roundel, but by a special name according to the tincture. Also roundle, roundlet.

In medieval armor: A round shield made of osiers, wood, sinews, or ropes covered with leather, or plates of metal, or stuck full of nails in concentric circles or other figures: sometimes made wholly of metal, and generally convex, but sometimes concave, and both with and without the umbo or boss. A piece of metal of circular or nearly circular form. A very small plate sewed or riveted to cloth or leather as part of a coat of fence. (β) A larger plate, used to protect the body at the défaut de la cuirasse, where that on the left side was fixed, that on the right side movable to allow of the couching of the lance, and at the knee-joint, usually one on each side, covering the articulation. Also called disk.

In fortification, a bastion of a semicircular form, introduced by Albert Dürer. It was about 300 feet in diameter, and contained roomy casemates for troops.

In architecture, a molding of semicircular profile.