A cross section of a solid is mathematically defined as the plane surface that results from the intersection of a plane with the solid. Therefore, a cross section is an infinitesimally thin slice of the solid. Solids commonly encountered in solid geometry include the sphere, ellipsoid, cylinder, cube, cone, and pyramid. Because solids are 3-dimensional figures, they have volumes that can be calculated. A cross section of a solid is a 2-dimensional shape, for which an area (called the cross-sectional area of the solid) can be calculated.

Although a cross section of a solid is mathematically a "slice" of the solid, it is easier to visualize a cross section as one of the cut ends produced when the solid is cut completely in two. For example, imagine that the earth were cut in two along the equator. Each...