Frae Wikipedia, the free beuk o knawledge
Circle wi circumference C in black, diameter D in cyan, radius R in reid, an centre or origin O in magenta.

In geometry, a radius o a raing or speere is a line segment frau its mid pynt til its perimeter. A radius is hal the lenth o the diameter in the same raing or speere. The radius o a regular polygon is the distance frau the pynt to a vertex.