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In geometry, the **semidiameter** or **semi-diameter** of a set of points may be one half of its diameter; or, sometimes, one half of its extent along a particular direction.

## Special cases

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.