A circle is defined as the set of all points in a plane that are the same distance from another point, which is called the center of the circle. When these points are connected, they form a closed curve. The equation for a circle on the Cartesian coordinate plane is x squared plus y squared equals r squared, where r is the radius of the circle.

The radius of a circle is a line segment that starts at the center and touches any point on the circle. Since all points on a circle are an equal distance from the center, the radius of a circle is constant.

The diameter of a circle is twice the length of the radius and passes through the center of the circle. It represents the longest line segment that can be contained inside a circle.

The circumference of a circle is the length around the circle. The ratio of the circumference of a circle to the diameter of a circle is the irrational number Pi, which is approximately 3.14. To calculate the circumference of a circle, multiply the diameter by Pi.

The area of a circle is calculated by taking the square of the radius and multiplying by Pi.