The word rondel is derived from Old French and means 'round' or 'small circle'.
Some weapons had guards that were given this name in reference to the circular shape of said guard. Particularly notable here are the pole axe and the aptly named rondel dagger.
The roundel of heraldry is derived from this word.
Decorative circles of embroidery or woven into fabric are often called rondells or decorative rondells.