Radius is a line from the center to the outside of a circle or sphere.(noun)
An example of radius is the spoke a bike wheel.
The definition of a radius is a circular limit or a boundary of a specific distance which is drawn from a specific point.(noun)
An example of radius is a restaurant located within five miles of a house.
See radius in Webster's New World College Dictionary
Origin: L, rod, spoke (of a wheel), hence radius, ray (of light), ? akin to radix, root
See radius in American Heritage Dictionary 4
noun pl. ra·di·i (-dē-īˌ) or ra·di·us·es
Origin: Latin, ray, spoke of a wheel, radius.
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