ringshaped

Ring-shaped is an adjective used to describe objects whose form includes a circular band with a central opening. In geometry, a two-dimensional ring is called an annulus, the region between two concentric circles. In three dimensions, a ring-shaped object can be a torus or a hollow circular body whose cross-section is circular.

A ring in two dimensions has area π(R^2 − r^2) where R is the outer radius and r is the inner radius. In three dimensions, ring-shaped objects include the torus (the surface generated by revolving a circle around an axis external to the circle) and hollow, donut-like shapes such as cylinders with a circular cross-section.

Rings appear across disciplines. In biology, ring-shaped cells and structures occur in various organisms. In chemistry, many cyclic compounds feature ring structures, and ring strain can influence reactivity. In physics and engineering, ring-shaped components such as bearings, seals, and resonators are common, and ring-shaped detectors and particle accelerators are used in experiments. In astronomy, planetary rings are prominent ring-shaped features around some planets.

Ring-shaped is closely related to, but distinct from, circular and annular terms. Circular typically denotes a solid disk, while annular describes the region between two concentric circles; ring-shaped emphasizes a hollow center but can also describe a three-dimensional toroid. In everyday language, ring-shaped is a descriptive, general term applicable to both geometric shapes and real-world objects, from decorative rings to engineering components.