The number pi, usually written with the Greek letter $\pi ,$ is a constant defined as the ratio between the circumference and the diameter of a circle. This ratio is the same for all circles, which is due to the fact that all circles are similar. Since $\pi$ is an irrational number, its decimal part never repeats or terminates. However, its value is often rounded to $3.14$ to make calculations easier. Alternatively, $\pi$ can be approximated by $\frac{22}{7}.$ Graphically, $\pi$ is the number of times that the diameter of the circle fits on top of the circle.