pi

Ratio of the circumference of a circle to its diameter. Pi is an irrational number: it cannot be expressed as the ratio of two integers, and its expression as a decimal never terminates and never starts recurring. The value of pi is 3.1415926, correct to seven decimal places. Common approximations to pi are 22/7 and 3.14, although the value 3 can be used as a rough estimation.

For example, a piece of string tied around the circumference of a mug will measure approximately three times the diameter of the mug.

Pi is also a transcendental number (a number that cannot be expressed as a root or as the solution of an algebraic equation with rational coefficients). This was proved in 1882 by German mathematician Ferdinand von Lindemann, and it means that from a line segment drawn to represent the diameter of a circle it is impossible to construct another straight-line segment to represent the circumference of the circle using straight edge and compasses only.