Definition of a circle:
In a plane, a closed curve formed by rotating around a point for a certain length is called a circle. On the plane, a circle is a set of points whose distance from a fixed point is equal to a fixed length. A circle has countless axes of symmetry, which pass through the center of the circle. A circle is a conic curve, which is obtained by cutting a cone from a plane parallel to the bottom of the cone.
The circle is defined as 360, because the ancient Babylonians moved one position every 4 minutes and 360 positions every 24 hours, so the inner angle of the circle is defined as 360. This degree represents the sun.
A circle is a geometric figure. By definition, compasses are usually used to draw circles. The diameter and radius of the inner circle of the same circle are always the same, and the circle has countless radii and diameters. A circle is a figure with axial symmetry and central symmetry.
The axis of symmetry is the straight line where the diameter lies. At the same time, the circle is a "positive infinite polygon" and "infinity" is just a concept. A circle can be regarded as a regular polygon composed of countless infinitesimal points. The more sides a polygon has, the closer its shape, perimeter and area are to a circle. So there is no real circle in the world, and the circle is actually just a conceptual figure.