Arcus

noun

1. In anatomy, an arch, bow, or ring.
2. A white band of cholesterol that forms at the edge of the cornea
3. A low, horizontal cloud typically forming at the leading edge of thunderstorm outflow
4. An elastic band around the base of the arolium, a pad at the end of the leg of certain insects
5. An arc-shaped band of thickened sexine extending between two apertures on a pollen grain or spore