This document provides definitions and explanations related to spheres. It defines a sphere as a solid generated by revolving a semicircle about its diameter. It lists examples of spheres from daily life like balls and globes. It explains that the diameter of a sphere passes through its center and that the center and radius of the semicircle used to generate the sphere are also the center and radius of the sphere. It defines a hemisphere as half of a sphere divided by a plane passing through the center. It provides formulas for the surface area of a hemisphere and sphere and the volume of a sphere and hemisphere.