radius

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Mathematics A line segment that joins the center of a circle with any point on its circumference.
2. n. Mathematics A line segment that joins the center of a sphere with any point on its surface.
3. n. Mathematics A line segment that joins the center of a regular polygon with any of its vertices.
4. n. Mathematics The length of any such line segment.
5. n. A circular area measured by a given radius: every family within a radius of 25 miles of the city center.
6. n. A bounded range of effective activity or influence: the operating radius of a helicopter.
7. n. A radial part or structure, such as a mechanically pivoted arm or the spoke of a wheel.
8. n. Anatomy A long, prismatic, slightly curved bone, the shorter and thicker of the two forearm bones, located on the lateral side of the ulna.
9. n. Anatomy A similar bone in many vertebrates.

Century Dictionary and Cyclopedia

1. n. In mathematics, one of a number of lines proceeding from a center; a ray; especially, a line drawn from the center to the periphery of a circle or sphere; also, the measure of the semidiameter.
2. n. In anatomy and zoology, the outer one of the two bones of the forearm, or corresponding part of the fore leg; the bone on the thumb side of the forearm, extending from the humerus to the carpus, and bearing upon its distal end the manus or hand: so called from its revolving, somewhat like a spoke, about the ulna, as in man and other mammals whose fore limb exhibits the motions called pronation and supination. In most animals, however, the radius is motionless, being fixed in a state of pronation, when it appears as the inner rather than the outer of the two bones, or as by far the larger bone, of the forearm, the ulna being often much reduced. In man the radius is as long as the ulna without the olecranon, and somewhat stouter, especially in its distal parts. It presents a small, circular, cupped and button-like head, for articulation with the capitulum of the humerus and lesser sigmoid cavity of the ulna, following which is a constriction termed the neck, and next to this a tubercle for the insertion of the biceps muscle. The shaft enlarges from above downward, and is of somewhat prismatic form, with the sharpest edge of the prism presenting toward the ulna. The lower end has two large articular facets for articulation with the scaphoid and lunar bones (forming the radiocarpal articulation, or wrist-joint), a lateral facet for the radio-ulnar articulation, and a stout projection called the styloid process, for the insertion of the supinator lougus muscle. The radius is pronated by the pronator radii teres and pronator quadratus, and supinated by the supinator longus and supinator brevis, assisted by the biceps. Quite a similar form and disposition of the radius characterize various mammals which use their fore paws like hands, as monkeys, mice, squirrels, opossums, etc. The radius of others, as the horse and ox, is more different, and associated with a much reduced and ankylosed ulna. In birds the radius is so peculiarly articulated with the humerus that it slides lengthwise back and forth upon the ulna in the opening and closing of the wing, pronation and supination being absent in this class of animals. See pronation and supination, and cuts under carpus, Catarrhina, Equidæ, forearm, ox, pinion, Plesiosaurus, and solidungulate.
3. n. In ichthyology, a bone of the pectoral arch, wrongly identified by some naturalists with the radius of higher vertebrates. The one so called by Cuvier is the hypercoracoid, and that of Owen is the hypocoracoid.
4. n. In entomology, a vein of the wing of some insects, extending from the pterostigma to the tip of the wing.
5. n. In conchology, a genus of Ovulidæ. R. volra is the shuttle-shell or weaver-shell.
6. n. plural In ornithology, the barbs of the main shaft of a feather; the rays of the first order of the rachis.
7. n. In arachnology, one of the radiating lines of a geometrical spider's web, which are connected by a single spiral line.
8. n. In echinoderms, one of the five radial pieces of the dentary apparatus of a sea-urchin, being an arched rod-like piece articulated at its base with the inner extremity of each rotula, running more or less nearly parallel with the rotula, and ending in a free bifurcated extremity. Also called the compass of the lantern of Aristotle (which see, under lantern). See also cut B under lantern.
9. n. plural Specifically, in Cirripedia, the lateral parts of the shell, as distinguished from the paries, when they overlap: when overlapped by others, they are called alæ.
10. n. In botany, a ray, as of a composite flower, etc.
11. n. The movable limb or arm of a sextant; also, a similar feature in any other instrument for measuring angles.
12. n. In fortification, a line drawn from the center of the polygon to the end of the outer side.
13. n. One of the principal longitudinal veins in an insect's wing, between the subcosta and the præmedia. It is vein III of Comstock's system.

Wiktionary

1. n. anatomy The long bone in the forearm, on the side of the thumb.
2. n. zoology The lighter bone (or fused portion of bone) in the forelimb of an animal.
3. n. geometry A line segment between any point on the circumference of a circle and its center/centre.
4. n. geometry The length of this line segment.

GNU Webster's 1913

1. n. (Geom.) A right line drawn or extending from the center of a circle to the periphery; the semidiameter of a circle or sphere.
2. n. (Anat.) The preaxial bone of the forearm, or brachium, corresponding to the tibia of the hind limb. See Illust. of Artiodactyla.
3. n. (Bot.) A ray, or outer floret, of the capitulum of such plants as the sunflower and the daisy. See Ray, 2.
4. n. The barbs of a perfect feather.
5. n. Radiating organs, or color-markings, of the radiates.
6. n. The movable limb of a sextant or other angular instrument.

WordNet 3.0

1. n. support consisting of a radial member of a wheel joining the hub to the rim
2. n. the outer and slightly shorter of the two bones of the human forearm
3. n. a circular region whose area is indicated by the length of its radius
4. n. the length of a line segment between the center and circumference of a circle or sphere
5. n. a straight line from the center to the perimeter of a circle (or from the center to the surface of a sphere)