Semidiameters refer to half the diameter of a circle or sphere. There are several synonyms that can be used for this term, including radius, half-diameter, and half-radius. The radius describes the distance from the center of a circle or sphere to its edge, while the semidiameter is half of the distance. Similarly, the half-diameter gives the length of the radius of the circle or sphere, while the half-radius indicates the distance from the center to an edge. These terms are useful in mathematical calculations, engineering and architecture, and even in astronomy. All in all, these synonyms are essential concepts that help us measure and communicate measurements more effectively.