Solid Geometry, branch of geometry that deals with the properties and measurement of geometric figures in three-dimensional space. Such figures include the cone, cube, cylinder, pyramid, sphere, and prism. Solid geometry extends and reinforces the propositions of plane geometry and is the necessary foundation for spherical trigonometry, solid analytical geometry, descriptive geometry, and other branches of mathematics. It is widely used in mathematics, engineering, and the natural sciences. See Geometry; Trigonometry.