Semidiameter

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.

pecial 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:$left(frac\{x\}\{a\}\; ight)^\{2\}+left(frac\{y\}\{b\}\; ight)^\{2\}=1;,!$Likewise, the semi-diameters of an ellipsoid are the parameters "a", "b", and "c" of its implicit equation:$left(frac\{x\}\{a\}\; ight)^\{2\}+left(frac\{y\}\{b\}\; ight)^\{2\}+left(frac\{z\}\{c\}\; ight)^\{2\}=1;,!$The semi-diameters of a superellipse, superellipsoid, or superquadric can be identified in the same way.