Image source: http://members.madasafish.com/~beardmore/Images/Reliant%20images/18%20rear%20end%20behind%20body.jpg
Circular objects that rotate often have an axis of rotation that pass through
their center. For example, the axis of a car passes through the center of a
tire in order to help the wheels turn so that the car can move.
Image source: http://williamlaird.com/CD%20%20PLAYER%20inside%20final.jpg
The motor of a compact disk player runs from its center to help the disk spin.
This carnival ride is in the shape of a circle. The red structure in the middle is the center of the circle. All points on the outside of the ride are the same distance from the center of the circle.