IN MATH: 1. n. a line segment having the circle's (or sphere's) center as one endpoint and a point on the circle (or sphere) as the other endpoint. 2. n. distance from the center of the circle (or sphere) to any point on the circle (or sphere). EX. Is the earth's radius 4,000 miles?

IN ENGLISH: 1. as defined above. 2. the distance from the center of a circular thing to its outside.