roundel

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A curved form, especially a semicircular panel, window, or recess.
• noun A rondel.
• noun A rondeau.

from The Century Dictionary.

• noun 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.
• noun Anything round; a round form or figure; a circle, or something of circular form.
• noun 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.
• noun 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.
• noun 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.
• noun In architecture, a molding of semicircular profile.
• noun A fruit-trencher of circular form.
• noun A dance in which the dancers form a ring or circle. Also called round.
• noun Same as rondel: specifically applied by Swinburne to a form apparently invented by himself.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Mus.) A rondelay.
• noun Anything having a round form; a round figure; a circle.
• noun A small circular shield, sometimes not more than a foot in diameter, used by soldiers in the fourteenth and fifteenth centuries.
• noun (Her.) A circular spot; a sharge in the form of a small circle.
• noun (Fort.) A bastion of a circular form.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Anything having a round form; a round figure; a circle.
• noun music A roundelay or rondelay.
• noun A small circular shield, sometimes not more than a foot in diameter, used by soldiers in the fourteenth and fifteenth centuries.
• noun heraldry A circular spot; a charge in the form of a small coloured circle.
• noun aviation a circular insignia painted on an aircraft to identify its nationality or service.
• noun A bastion of a circular form.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (heraldry) a charge in the shape of a circle
• noun round piece of armor plate that protects the armpit
• noun English form of rondeau having three triplets with a refrain after the first and third