Radii n. : pl. of Radius.

Radii pl. : of Radius

Example:
What are the inner and outer Radii of the shell?

Taxonomy
Radius, radius of a circle, Formulas, circle, 08-08, free encyclopedia, regular polygon, the distance, three points, hypercubes, maximum radius, minimum radius, circumradius, inradius, vertices, classical geometry, inner radius, circumference.

Defination from the web:
radius of a circle or sphere is any line segment from its center to its perimeter. ... In either case, the radius may be more than half the diameter (which is usually defined as the maximum distance between any two points of the figure) ...