/ ˈreɪdiəs; NAmE / noun ( pl. -dii / -diaɪ; NAmE /)
1.
a straight line between the centre of a circle and any point on its outer edge; the length of this line
—compare diameter —picture at circle
2.
a round area that covers the distance mentioned from a central point :
They deliver to within a 5-mile radius of the store.
3.
( anatomy ) the shorter bone of the two bones in the lower part of the arm between the elbow and the wrist, on the same side as the thumb
—picture at skeleton
—see also ulna
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WORD ORIGIN
late 16th cent. (in sense 3): from Latin , literally staff, spoke, ray .