The semi-diameter of a sphere, circle, or interval is the same thing as its radius — namely, any line segment from the center to its boundary.
The semi-diameters of a non-circular ellipse are the halves of its extents along the two axes of symmetry. They are the parameters a, b of the implicit equation
Likewise, the semi-diameters of an ellipsoid are the parameters a, b, and c of its implicit equation
The semi-diameters of a superellipse, superellipsoid, or superquadric can be identified in the same way.