noun ra·di·us \ˈrā-dē-əs\

: a straight line from the center of a circle or sphere to any point on the outer edge; also : the length of this line

: an area that goes outward in all directions from a particular place

medical : the bone in the lower part of your arm on the same side as your thumb

plural ra·dii \-dē-ˌī\ also ra·di·us·es

Full Definition of RADIUS

:  a line segment extending from the center of a circle or sphere to the circumference or bounding surface
a :  the bone on the thumb side of the human forearm; also :  a corresponding part of vertebrates above fishes
b :  the third and usually largest vein of an insect's wing
a :  the length of a radius <a truck with a short turning radius>
b :  the circular area defined by a stated radius
c :  a bounded or circumscribed area
:  a radial part
:  the distance from a center line or point to an axis of rotation

Examples of RADIUS

  1. Measure the radius of the circle.
  2. a radius of 10 inches
  3. The new museum is expected to draw people from a wide radius.
  4. There are three restaurants within a one-block radius of our apartment.

Origin of RADIUS

Latin, ray, radius
First Known Use: circa 1611

Other Mathematics and Statistics Terms

abscissa, denominator, divisor, equilateral, exponent, hypotenuse, logarithm, oblique, radii, rhomb
RADIUSES Defined for Kids


noun ra·di·us \ˈrā-dē-əs\
plural ra·dii \-dē-ˌī\

Definition of RADIUS for Kids

:  a straight line extending from the center of a circle to the outside edge or from the center of a sphere to the surface
:  an area that extends in all directions from a place <Most students live within a radius of five miles from the school.>
:  the bone on the thumb side of the arm between the wrist and the elbow
Medical Dictionary


noun ra·di·us \ˈrād-ē-əs\
plural ra·dii \-ē-ˌī\ also ra·di·us·es

Medical Definition of RADIUS

:  the bone on the thumb side of the human forearm or on the corresponding part of the forelimb of vertebrates above fishes that in humans is movably articulated with the ulna at both ends so as to permit partial rotation about that bone, that bears on its inner aspect somewhat distal to the head a prominence for the insertion of the biceps tendon, and that has the lower end broadened for articulation with the proximal bones of the carpus so that rotation of the radius involves also that of the hand

Illustration of RADIUS


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