A circle is a set of all points in a plane that are equidistant from a fixed point called the center. The distance from the center to any point on the circle is called the radius.
Ellipse: A set of all points in a plane such that the sum of their distances from two fixed points (foci) is constant.
Radius: The distance from the center of a circle to any point on its circumference.
Center: The fixed point equidistant from all points on a circle.