A curved form, especially a semicircular panel, window, or recess.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "roundel" in a sentence
"Another piece of information I dug out as I was reading about the roundel is that it was traditionally used by officers of arms and the military to signify nationality."
"Above the two in the roundel is another Saint Michael bravely slaying a fire-breathing dragon."
"Two new ones are the hex and pentagon, while the circle (once called a roundel, bezant, plate, torteau, hurt, etc., depending on its color) is simply called a circle with the correct color named."