Definition:Circle/Radius

This page is about radius in the context of circle. For other uses, see radius.

Definition

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.

Also defined as

The radius of a circle can also be used to refer to the length of the line from the center to the circumference.

Also see

• Results about radii of circles can be found here.

Linguistic Note

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.

The adjectival form radial usually means directed along a radius.

Sources

• 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): radius: 2. (plural radii)
• 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): radius: 2. (plural radii)
• 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): radius (radii)