The zone of a sphere, also called spherical zone, or simply zone, is the spherical surface of the sphere between two parallel planes cutting the sphere.
In the figure above, we show two spherical zones using the color red and the color green.
A spherical zone can also have just 1 plane. In this case, it is the spherical surface between 1 plane and the end of the sphere.
In the figure below, the zone of the sphere is shown with the color orange.