The circumference is the perimeter of the circle. An arc is a part of the circumference. A major arc is greater than half the total circumference, a minor arc is less than half the circumference.

The circumference of a circle is, in turn ... were de rigeur in mathematics for another millennium and a half—that is, until calculus swept in and stole the show. The earliest recorded use ...

which means you can figure out the circumference of any circle if you know its diameter or radius (half the diameter) and pi, which is most commonly abbreviated as 3.14 — which is why Pi Day is ...