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This page is about Radius in the context of Circle. For other uses, see Radius.
A radius of a circle is a straight line segment whose endpoints are the center and the circumference of the circle.
In the above diagram, the line $AB$ is a radius.
The plural of radius is radii, pronounced ray-dee-eye.
This irregular plural form stems from the Latin origin of the word radius, meaning ray.
The ugly incorrect form radiuses can apparently be found, but rarely in a mathematical context.
- 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): radius (radii)