noun

geometrya shape where every point along its perimeter is an equal distance from its center."the circle with a radius of one is called the unit circle."
A circle is a shape where every point along its perimeter is an equal distance from its center.
Since a circle is used throughout math, many of its properties and attributes are given specific names.
Attributes  

Radius The radius is the distance from the center of the circle to any point along the perimeter of the circle. 

Circumference The circumference is the length around the perimeter of the circle. 

Diameter The diameter is the distance between any two points along the perimeter of the circle that passes through the center of the circle. 

Area The area of a circle is the amount of twodimensional space that fits inside the perimeter of the circle. 

Circle Constant The circle constant (tau) is a naturally occuring number calculated by dividing the circumference of any circle by its radius. 
A sphere is a threedimensional geometric shape that is perfectly symmetrical around its center. All points on the surface of a sphere are equidistant from its center.