A two-dimensional geometric figure (a collection of points) in three-dimensional space. The simplest example is a plane—a flat surface. Some other common surfaces are spheres, cylinders, and cones, the names of which are also used to describe the three-dimensional geometric figures that are enclosed (or partially enclosed) by those surfaces. In a similar way, cubes, parallepipeds, and other polyhedra are surfaces. Often only the context indicates whether a surface or a solid is being referred to, but in modern mathematical usage such words refer only to surfaces.