There is a proportional relationship between the diameter and circumference of any circle. That means that if we write $C$ for circumference and $d$ for diameter, we know that $C=kd$, where $k$ is the constant of proportionality.

The exact value for the constant of proportionality is called pi, and its symbol is $\boldsymbol\pi$. Some frequently used approximations for $\pi$ are $\frac{22} 7$, 3.14, and 3.14159, but none of these is exactly $\pi$.