Definition: A type of helment comprised of a snug-fitting bowl that came to just above the ears, fitted with cheek plates that attached by hinges to this bowl. It often came with chin pieces that clasped in the front and secured in the back along a strip of steel that extended from the shallow skull bowl.

“Armets à rondel were the dominant helmet during much of the 15th century.”

Source: https://www.medievalwarfare.info/glossary.htm