A circle is one of those really simple mathematical objects which unlock a treasure trove of results. All you need to define a circle is a fixed point and a given length: if you collect together all the points which are that given length away from the fixed point, you have a circle. The fixed point is called the center, and the fixed length is called the radius of the circle. If you double the radius, you have the diameter of the circle, and the length of the circular path going once around the circle is its circumference. The diameter, then, is the longest straight line you can draw between two points on the circumference of the circle, and it goes through the center.