radius

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A line segment that joins the center of a circle with any point on its circumference.
• noun A line segment that joins the center of a sphere with any point on its surface.
• noun A line segment that joins the center of a regular polygon with any of its vertices.
• noun The length of any such line segment.
• noun A circular area measured by a given radius.
• noun A bounded range of effective activity or influence.
• noun A radial part or structure, such as a mechanically pivoted arm or the spoke of a wheel.
• noun A long, prismatic, slightly curved bone in humans, the shorter and thicker of the two forearm bones, located on the lateral side of the ulna.
• noun A similar bone in many other vertebrates.

from The Century Dictionary.

• noun 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.
• noun 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.
• noun 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.
• noun 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.
• noun In entomology, a vein of the wing of some insects, extending from the pterostigma to the tip of the wing.
• noun In conchology, a genus of Ovulidæ. R. volra is the shuttle-shell or weaver-shell.
• noun plural In ornithology, the barbs of the main shaft of a feather; the rays of the first order of the rachis.
• noun In arachnology, one of the radiating lines of a geometrical spider's web, which are connected by a single spiral line.
• noun 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.
• noun 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.
• noun In botany, a ray, as of a composite flower, etc.
• noun The movable limb or arm of a sextant; also, a similar feature in any other instrument for measuring angles.
• noun In fortification, a line drawn from the center of the polygon to the end of the outer side.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geom.) A right line drawn or extending from the center of a circle to the periphery; the semidiameter of a circle or sphere.
• noun (Anat.) The preaxial bone of the forearm, or brachium, corresponding to the tibia of the hind limb. See Illust. of Artiodactyla.
• noun (Bot.) A ray, or outer floret, of the capitulum of such plants as the sunflower and the daisy. See Ray, 2.
• noun The barbs of a perfect feather.
• noun Radiating organs, or color-markings, of the radiates.
• noun The movable limb of a sextant or other angular instrument.
• noun (Mach.) a bar pivoted at one end, about which it swings, and having its other end attached to a piece which it causes to move in a circular arc.
• noun See under Curvature.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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