a circle is a shape in which all points are equidistant from a point in the middle called the "center".
the diameter of the circle is the distance across the circle that passes through two points on the the circle and the center of the circle. the diameter is the largest distance between any two points on the circle. the radius is half of the diameter.
the length of the distance around the circle is known as the perimeter.
the ratio of a circle's circumference to it's diameter is pi. pi is approximaately 3.141592654. this is a constant and is true of all circles.
