concentric

Concentric describes objects that share a common center. In geometry, the term is most often applied to circles, where two or more circles are concentric if they have the same center but different radii. The concept also extends to spheres, cylinders, or any set of figures arranged around a common point, producing rings or shells with increasing or decreasing radii.

Etymology: from Latin con- “together” and centrum “center,” the word is used to denote layers or rings that share a center. It entered English with the sense of layers arranged around a central point.

Properties: Concentric figures do not intersect each other; they are centered on a single point. Each circle or sphere has a constant radius from the common center, and the distance between any two concentric figures is the difference of their radii. Concentricity implies radial symmetry around the shared center.

Examples: Concentric circles appear in many contexts, such as bullseye targets, clock faces, and decorative patterns. They also describe onion layers and tree growth rings, where successive shells surround a central core. In urban geography, the concentric zone model describes city expansion outward from a central business district. In design and engineering, concentric rings are used in decorations, gaskets, seals, and mechanical components.

See also: coaxial, annulus, rings, radial symmetry.