Geometrically, a figure composed of all points whose distance from a plane to a fixed point is equal to a fixed length is called a circle. A fixed point is called the center of the circle and a fixed length is called the radius.

Trajectory theory: the trajectory of a moving point on a plane with a certain length as the center is called a circle.

Set theory: The set of points whose distance to a fixed point is equal to a fixed length is called a circle.