Definition of an ellipse in terms of its foci:
An ellipse is defined to be a locus of points such that the sum of the distance of any point from two fixed points is always constant.
These two fixed points are called foci of an ellipse.
The special case of the ellipse is the circle.
When the major and minor axis is of the same length then it must be the diameters of a circle.
Thus, the special case of the ellipse is the circle.