noun, plural ra·di·i [rey-dee-ahy] /ˈreɪ diˌaɪ/, ra·di·us·es.

  1. a straight line extending from the center of a circle or sphere to the circumference or surface: The radius of a circle is half the diameter.
  2. the length of such a line.
  3. any radial or radiating part.
  4. a circular area having an extent determined by the length of the radius from a given or specified central point: every house within a radius of 50 miles.
  5. a field or range of operation or influence.
  6. extent of possible operation, travel, etc., as under a single supply of fuel: the flying radius of an airplane.
  7. Anatomy. the bone of the forearm on the thumb side.Compare ulna(def 1).
  8. Zoology. a corresponding bone in the forelimb of other vertebrates.
  9. Machinery Now Rare. the throw of an eccentric wheel or cam.
  10. a rounded corner or edge on a machined or cast piece of metal.
  11. Entomology. one of the principal longitudinal veins in the anterior portion of the wing of an insect.

noun plural -dii (-dɪˌaɪ) or -diuses

  1. a straight line joining the centre of a circle or sphere to any point on the circumference or surface
  2. the length of this line, usually denoted by the symbol r
  3. the distance from the centre of a regular polygon to a vertex (long radius) or the perpendicular distance to a side (short radius)
  4. anatomy the outer and slightly shorter of the two bones of the human forearm, extending from the elbow to the wrist
  5. a corresponding bone in other vertebrates
  6. any of the veins of an insect’s wing
  7. a group of ray florets, occurring in such plants as the daisy
    1. any radial or radiating part, such as a spoke
    2. (as modifier)a radius arm
  8. the lateral displacement of a cam or eccentric wheel
  9. a circular area of a size indicated by the length of its radiusthe police stopped every lorry within a radius of four miles
  10. the operational limit of a ship, aircraft, etc

1590s, “cross-shaft,” from Latin radius “staff, stake, rod; spoke of a wheel; ray of light, beam of light; radius of a circle,” of unknown origin. Perhaps related to radix “root,” but Tucker suggests connection to Sanskrit vardhate “rises, makes grow,” via root *neredh- “rise, out, extend forth;” or else Greek ardis “sharp point.”

The geometric sense first recorded 1610s. Plural is radii. Meaning “circular area of defined distance around some place” is attested from 1953. Meaning “shorter bone of the forearm” is from 1610s in English (the Latin word had been used thus by the Romans).

n. pl. ra•di•us•es

  1. A line segment that joins the center of a circle with any point on its circumference.
  2. A long, prismatic, slightly curved bone, the shorter and thicker of the two forearm bones, located laterally to the ulna.

Plural radii (dē-ī′) radiuses

  1. A line segment that joins the center of a circle or sphere with any point on the circumference of the circle or the surface of the sphere. It is half the length of the diameter.
  2. The shorter and thicker of the two bones of the forearm or the lower portion of the foreleg. See more at skeleton.

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