Pi
Pi (denoted by the Greek letter $\pi$) is a mathematical constant representing the ratio of a circle's circumference to its diameter. It is an irrational number, meaning its decimal representation never ends and never repeats.
Example
For practical calculations in secondary school, $\pi$ is often approximated as $\frac{22}{7}$ or $3.14159$. The circumference $C$ of a circle with a diameter $d = 14$ cm is $C = \pi d \approx \frac{22}{7} \times 14 = 44$ cm.