ENGLISH
REFERENCE

radius

n. · v.
US //ˈɹeɪdiəs// UK //ɹˈeɪdɪəs// ra·dius
  1. 1 math/geometry (n.)
    B2 Upper Intermediate Math

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

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

    Example

    The teacher asked us to calculate the area of a circle with a radius of five centimeters.

    Example

    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. 2 area around a point (n.)
    B2 Upper Intermediate

    the area within a certain distance of a specific place.

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

    Example

    There are at least three good coffee shops within a one-mile radius of my house.

    Example

    The emergency services established a five-kilometer radius around the site of the chemical spill to ensure public safety.
  3. 3 anatomy (n.)
    C1 Advanced Technical Medicine

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

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

    Example

    He fell off his bike and broke his radius, so he has to wear a cast for six weeks.

    Example

    The X-ray revealed a hairline fracture of the distal radius, necessitating a specialized splint to ensure proper alignment during the healing process.
    • ulna
  4. 4 to round an edge (v.)
    C2 Proficiency Technical

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

    to give a rounded or curved profile to an edge or corner.

    Example

    The carpenter decided to radius the corners of the table so they wouldn't be so sharp.

    Example

    The design specifications require the machinist to radius all external edges to three millimeters to prevent stress fractures in the alloy.
Origin

Borrowed from Latin radius (“ray”). Doublet of ray.

Etymology adapted from Wiktionary, available under CC BY-SA 4.0.

Related words

Last updated June 20, 2026