Pi

$\pi = \frac {C}{d}$ (pi is equal to the circumference divided by the diameter).

Pi is an endless string of numbers.
Pi or $\pi$ is a mathematical constant, one of the first ever discovered. It is the ratio of the distance around a circle to the circle's diameter. This produces a number, and that number is always the same. However, the number is rather strange. The number starts 3.14159265... and continues without end. Such numbers are called irrational.
The diameter is the longest line which can be fitted inside a circle. It passes through the center of the circle. The distance around a circle is known as the circumference. Even though the diameter and circumference are different for different circles, the number pi remains constant: its value never changes. This is because the relationship between the circumference and diameter is always the same.

This diagram showing how PI can be found by using a circle with a diameter of one . The circumference of this circle is  /Pi  .

See This Nice Animated Video about Pi

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