Definition of: radius

(rā′dē·əs) noun plural ·di·i (-dē·ī)
1. A straight line from the center of a circle or sphere to its periphery.
2. Anat. The thicker and shorter bone of the forearm, on the same side as the thumb.
3. Bot. A ray floret of a composite flower; also, a branch of an umbel.
4. Zool. a In radiolarians and similar organisms, the imaginary line or plane dividing the body into two theoretically equal parts. b A ray or radiating part, as, the barb of a feather. c A lateral part of a cirriped shell when overlapping others.
5. Entomol. One of the main longitudinal veins of an insect's wings.
6. In a sextant, quadrant, etc., a pivoted arm, mounted so as to move radially, as on a graduated arc or circle.
7. Mech. A wheel spoke; a rod or bar which with others extends from a common point.
8. A circular area or boundary measured by the length of its radius.
9. Sphere, scope, or limit, as of activity.
10. A fixed limit of travel beyond which higher fares are charged. [<L, orig., rod, spoke of a wheel, hence radius, ray of light. Doublet of RAY]