Which defines a circle? two rays with a common endpoint a piece of a line with two endpoints a piece of a line with one endpoint all coplanar points equidistant from a given point

all coplanar points equidistant from a given point

This is another way of saying "the set of all points that are equally distant from a fixed point" (eg: the set of all points 5 units away from some fixed point). The "fixed point" is the center of the circle. The distance from the center to any point on the circle is the radius.