Diameter, in geometry, is any straight line segment that passes through the center of a circle or a sphere and touches the figure’s boundaries at opposite points. The word diameter also refers to the length of such a line segment.
The diameter of a circle or sphere is twice as long as the figure’s radius. The radius is any line segment that runs from the center of a circle or a sphere to any point on the figure’s boundary. Radius also refers to the length of such a line segment.
The degree of magnification achieved by a microscope or telescope is expressed in diameters. For example, a microscope that doubles the apparent size of an object is said to magnify the object two diameters.
See also Circle; Microscope; Sphere.
