Annulus is a term used to describe a RING-shaped object. It is derived from the Latin word annulus, which means 'little RING'. Annulus can refer to a variety of objects, such as a circular band or a hollow cylinder. In mathematics, annulus is used to describe a region bounded by two concentric circles. In biology, annulus is used to describe the RING-shaped structure of certain organisms, such as the annulus of a jellyfish. In astronomy, annulus is used to describe the RING-shaped structure of certain stars and galaxies. In engineering, annulus is used to describe the shape of certain components, such as the annular wing of an aircraft.