Radius

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The radius of a circle is the distance from the center to any point on the circle. Identical definitions hold for the sphere and hypersphere. The plural form of radius is radii.

$[asy]size(115); pair O=origin, P=dir(30); D(unitcircle); D(O--P); MP("O",D(O),NNW); MP("P",D(P),NE); MP("r",midpoint(O--P),NNW);[/asy]$