Radius

How to spell radius

6 letters · 3 syllables

How to pronounce radius

Say the part in capitals loudest.

What does radius mean?

1. nounThe straight distance from the centre of a circle or sphere to its edge.
2. nounA circular area measured outward from a central point.
3. nounThe shorter of the two long bones in the forearm, on the thumb side.

Radius in a sentence

• The engineer measured the radius of the curved pipe before cutting it.

• Every shop within a one mile radius delivers to our office.

• The x-ray showed a clean fracture in the radius near her wrist.

• To find the area of the circle, square the radius and multiply by pi.

• The storm caused flooding across a wide radius of the valley.

• The surgeon explained how the radius rotates over the other forearm bone.

Where does radius come from?

Language of origin: Latin

This word is taken directly from the Latin radius, which literally meant a 'staff', a 'rod', or the 'spoke of a wheel', and by extension a 'ray' of light. The Romans pictured the straight spokes running from the hub of a wheel out to its rim, which is exactly the line a mathematician draws from the centre of a circle to its edge, so the geometric sense was a natural fit. English adopted the term in the late 1500s, first for the bone of the forearm and soon afterwards for the geometric line, keeping the Latin spelling and the Latin plural radii. The same root gives us 'radiate', 'radiant', and even 'radio', all carrying the idea of something spreading out from a central point like rays or spokes. The spelling trap is the unstressed middle, since the -di- can sound like a vague 'dee' and the ending is -us, not -ous.

How the word is built

• radi- (from Latin radius: ray, spoke of a wheel)
• -us (Latin noun ending)