n. Something round.n. In metallurgy, one of successive crusts which form upon the surface of molten metal while cooling, and which as they form are removed for further treatment.n. Milit.: A small shield (15 inches in length) formerly used by pikemen and archers.n. One of the iron disks, each having an opening in the center for the passage of a bolt, placed between the cheeks and stock of a field-gun carriage in bolting these parts together.n. A semicircular bastion introduced by Albert Dū rer. It was about 300 feet in diameter, and contained spacious casemates.n. A disk of colored glass, used as a decoration in stained-glass windows. Compare bull's-eye, 8.