- (medicine) A white band of cholesterol that forms at the edge of the cornea
- (meteorology) A low, horizontal cloud typically forming at the leading edge of thunderstorm outflow
- (entomology) An elastic band around the base of the arolium, a pad at the end of the leg of certain insects
- (palynology, plural sometimes arci) An arc-shaped band of thickened sexine extending between two apertures on a pollen grain or spore
Borrowed from Latin arcus.