One definition of circumference is the boundary line of a circle and is related to π and the circle's diameter by the equation π=C/D where C is the circumference and D the diameter of the circle.
A second, more general definition is the boundary line of any closed curvilinear figure. Synonyms are periphery and perimeter.
Two equations for the circumference of a circle are C=πD=2πr where r is the radius. To get other equations, you need to specify the closed curve for which you want the perimeter.