The number Pi is a constant approximately equal to 3.14159 and it is defined as the ratio of a circle’s circumference to its diameter. It is represented by the Greek letter $\pi$.

Pi is an irrational number as it cannot be represented as ratio of integers.

The first references to the constant was by Welsh mathematician William Jones in his 1706 work Synopsis Palmariorum Matheseos; or, a New Introduction to the Mathematics. Jones’ notation was not immediately adopted by other mathematicians, with the fraction notation still being used as late as 1767.