Roundel
A roundel is a circular piece of glass set in a leaded panel. Many are cut from crown glass and have the thick, swirled bullseye in the middle, which gives a lens-like effect and catches the light. Others are painted discs, used in houses from the sixteenth century onward to carry heraldry, initials, scenes and decorative devices.
Roundels are often placed in a ring or a line in the borders of a door or window panel, or at the intersections of a lattice. Their circular shape contrasts with the straight lines around them, and they give a window a recognisably traditional look.
The terms roundel and rondel are used interchangeably, and a clear, thick roundel is often called a bullseye.