1. Definition of a circle: A circle is a closed figure composed of all points on a plane with the same distance from a given point (center).

2. Center and radius: the center of the circle is the center point of the circle, which is represented by the letter O; The radius is a line segment from the center of the circle to any point on the circle, which is represented by the letter R.

3. Diameter: The diameter is a line segment connecting two points on the circle through the center of the circle, and its length is twice the radius, which is represented by the letter D. ..

4. Arc chord: an arc is the curve between any two points on a circle; A chord is a straight line between any two points on a circle.

5. Sector: A sector is a figure consisting of the center, radius and arc between two points on the circle.

6. perimeter and area: the perimeter of a circle is the sum of the lengths of line segments from any point on the circle to the center of the circle, which is represented by the letter c; The area of a circle is the sum of the distances from all points in the circle to the center of the circle, which is represented by the letter A.

7. Tangent: Tangent is a straight line with only one intersection with the circle, and the tangent point is the intersection of tangent and the circle.

8. Isosceles triangle: An isosceles triangle refers to a triangle with two equal sides, one of which is the diameter.

9. Right triangle: A right triangle refers to a triangle with an angle of 90 degrees and a diameter on one side.

10. Curvature system: Curvature system is a unit used to express angles, and the angle of a circle is defined as 2π radians.

The above is some important knowledge about circles in primary school mathematics. By learning this knowledge, students can master the basic concept, nature and application of circle, and lay a foundation for further study of geometry.