‘直径’ (zhí jìng) is a mathematical noun meaning 'diameter' — the straight-line distance passing through the center of a circle or sphere, connecting two points on its boundary. The first character 直 (zhí) means 'straight' or 'direct', emphasizing the line’s unbroken, center-crossing nature; the second character 径 (jìng) means 'path' or 'route', here referring to a measured linear path across a shape. Together, they literally convey 'straight path' — precisely capturing the geometric definition.

This term is used primarily in mathematics, engineering, science, and everyday measurements involving circular objects — such as describing pipe sizes, wheel dimensions, or planetary data. It appears in textbooks, technical manuals, construction specs, and even casual contexts like buying a pizza ('这个披萨的直径是30厘米'). Unlike general size terms like 大小 (dàxiǎo), 直径 specifies an exact, measurable linear dimension unique to round or spherical forms.