‘半径’ (bàn jìng) is a mathematical noun meaning ‘radius’—the straight-line distance from the center of a circle (or sphere) to any point on its edge. The character 半 means ‘half’, and 径 means ‘diameter’ or ‘path’; together, they literally convey ‘half of the diameter’, which precisely defines the radius in geometry. This term is used exclusively in formal, academic, or technical contexts—especially in mathematics, physics, engineering, and design.

You’ll encounter 半径 in textbooks, diagrams, measurements, and technical instructions—for example, when calculating area (πr²), describing circular motion, or specifying dimensions of round objects like wheels, lenses, or planetary orbits. It’s not used in casual speech or everyday conversation unless discussing geometry or technical specifications. While it shares the character 径 with words like 直径 (zhí jìng, ‘diameter’) and 曲径 (qū jìng, ‘winding path’), 半径 has a strictly quantitative, scientific meaning.