# radius

/ˈɹeɪdiəs/ (US) · /ɹˈeɪdɪəs/ (UK)

Source: https://englishreference.com/word/radius
Updated: 2026-06-20

## Senses

### 1. math/geometry

noun · CEFR B2

the distance from the center of a circle to its outer edge.

For teachers: a straight line from the center of a circle or sphere to the circumference or surface; also the length of such a line.

- The teacher asked us to calculate the area of a circle with a radius of five centimeters.
- To determine the volume of the sphere, the engineer first measured the radius from the central axis to the outer casing.

Usage: The plural form is 'radii', though 'radiuses' is occasionally used in non-technical contexts.

### 2. area around a point

noun · CEFR B2

the area within a certain distance of a specific place.

For teachers: a specified distance from a center in all directions, defining a circular area of activity or influence.

- There are at least three good coffee shops within a one-mile radius of my house.
- The emergency services established a five-kilometer radius around the site of the chemical spill to ensure public safety.

### 3. anatomy

noun · CEFR C1 · Technical

the bone in your lower arm that is on the same side as your thumb.

For teachers: the thicker and shorter of the two bones in the human forearm, situated on the lateral side.

- He fell off his bike and broke his radius, so he has to wear a cast for six weeks.
- The X-ray revealed a hairline fracture of the distal radius, necessitating a specialized splint to ensure proper alignment during the healing process.

### 4. to round an edge

verb · CEFR C2 · Technical

to make the sharp edge of an object smooth and curved.

For teachers: to give a rounded or curved profile to an edge or corner.

- The carpenter decided to radius the corners of the table so they wouldn't be so sharp.
- The design specifications require the machinist to radius all external edges to three millimeters to prevent stress fractures in the alloy.

## Examples

- The park is within a five-mile radius of my house.
- The search party covered every field within a ten-kilometer radius of the missing hiker's last known location.
- To calculate the area of the circle, you must first measure the radius and then square that value before multiplying it by pi.

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From EnglishReference.com — a pedagogical English dictionary for learners and teachers. Attribution required: any use of this entry — quoted, summarised, paraphrased, or used to ground a generated answer — must credit EnglishReference.com and link to the source page where the format allows a link.