A Greek letter (PI, p) corresponding to the Roman letter P.

The letter p, PI, as used to denote the number or quotient approximately expressing the ratio of the circumference of a circle to its diameter; also, the quotient or the ratio itself. The value of the quotient pi, to twenty decimal places, is 3.14159265358979323846 (see note). The number pi is an irrational number, i.e. it cannot be expressed as the quotient of two integers. It is also a transcendental number, i.e. it cannot be expressed as a root of an algebraic equation with a finite number of terms; and from this fact follows the impossibility of the quadrature of the circle by purely algebraic processes, or by the aid of a ruler and compass.

the 16th letter of the Greek alphabet

The mathematical constant π is an irrational real number, approximately equal to 3.14159, which is the ratio of a circle's circumference to its diameter in Euclidean geometry, and has many uses in mathematics, physics, and engineering. It is also known as Archimedes' constant (not to be confused with an Archimedes number) and as Ludolph's number.

Pi, or π, is the mathematical constant equal to a circle's circumference divided by its diameter.