Formed when a plane intersects a cone perpendicular to the axis. It is the collections of all points, (x,y), which are equidistant from the center of the circle.  The radius of a circle is the distance between the center of a circle and a point on the circle. The distance from one end of the circle through the center to the other end of the circle is the diameter. Because all points on a circle are the same distance from the center, all radii are equal in length.  

Khan Academy Practice:

• Writing Standard Equation of Circle

• Graphing Circles from General Equation