I believe that is the definition of a straight line.
a straight line ..
A line in 2D and a plane in 3D A perpendicular bisector of the line connecting the 2 given points
A Circle.
you dont
A plane midway between the two given planes and parallel to them.
a straight line ..
A line in 2D and a plane in 3D A perpendicular bisector of the line connecting the 2 given points
A Circle.
A line that is the angle bisector.
you dont
The center of the circle. That's how the circle is defined. (The collection of all points on a plane equidistant from a fixed point. The fixed point is the center and the fixed distance is the radius.)
A plane midway between the two given planes and parallel to them.
That set of points forms what is known as a "circle".
It's another line, parallel to both of the first two and midway between them.
A point. In fact it is fixed point and the locus of all points, in a plane that is a fixed distance from that fixed point defines the circle.
The set of all points in the plane equidistant from one point in the plane is named a parabola.
All points on the circumference of a circle drawn on a plane are equidistant from the single point on the plane which is the center of the circle.