An ellipse also results from the intersection of a circular cone and a plane cutting completely through the cone. The maximum diameter is called the major axis. It determines the size of an ellipse. Half the maximum diameter, the distance from the center of the ellipse to one end, is called the semi-major axis. The shape of an ellipse is determined by how close together the foci are in relation to the major axis.
Eccentricity is the distance between the foci divided by the major axis.
If the foci coincide, the ellipse is a circle. Therefore, a circle is an ellipse with an eccentricity of zero.