Pi is the ratio of the circumference of a circle to its diameter. This value is a mathematical constant that has been calculated to over one trillion digits beyond its decimal point.

In some countries, such as the United Kingdom and Australia, Pi Approximation Day is celebrated on July 22nd (22/7) because 22/7 is a common approximation of pi.

Pi has many interesting properties, such as being a transcendental number, which means that it is not the solution to any non-zero polynomial equation with rational coefficients.