Description: Radius, diameter, center, and circumference all are parts of a circle. Let's go through each and understand how they are defined. Created by Sal Khan and Monterey Institute for Technology and Education. Draw a circle and label the radius, diameter, center, and the circumference. And it won't be that well drawn of a circle, but I think you get the idea. And actually, the circle itself is the set of all points that are a fixed distance away from that center.*Teacher Advisor is 100% free.