1. Diameter of a sphere: a segment passing through the center with endpoints on the sphere.
2. Congruent spheres: spheres that have equal radii.
3. Radius of sphere: a segment with endpoints at the center of the sphere and a point on the sphere.
4. Circle: the set of all points in a plane that are at a given distance from a given point in the plane.
5. Great circle: the intersection of a sphere and a plane containing the center of the sphere.
6. Small circle: the intersection of a sphere and a plane not containing the center of the sphere.
7. Sphere: the set of all points that are the same distance from a given point.
