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

The distance between each point on the circle and the center of the circle is equal; The distance to a fixed point is equal to a fixed length, and all points are on the same circle.

The three vertices of a triangle define a circle, which is called the circumscribed circle of the triangle. The center of the circumscribed circle of a triangle is the intersection of three perpendicular bisector of the triangle, which is called the outer center of the triangle.