Circumference
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Circumference is essentially a synonym for perimeter: for a given closed curve in the plane, it is the distance one travels in a complete circuit of the curve. The term circumference is most frequently used to refer to the distance around a circle, though it may refer to the distance around any smooth curve, while the term perimeter is typically reserved for polygons.
In a circle of radius 
and diameter 
, the circumference 
is given by 
. Indeed, the constant 
(pi) was originally defined to be the ratio of the circumference of a circle to the length of its diameter.
