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.
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( | x |
| a |
\right)2+\left(
| y | |
| b |
\right)2=1.
| \left( | x |
| a |
\right)2+\left(
| y | |
| b |
\right)2+\left(
| z | |
| c |
\right)2=1.