1841, used in L. 1748 by Swiss mathematician Leonhart Euler (1707-83), from Gk. letter pi (from Heb., lit. "little mouth") as an abbreviation of Gk. periphereia "periphery." For the meaning "printer's term for mixed type," see pie (3).
(pī) An irrational number that has a numerical value of 3.14159265358979... and is represented by the symbol π. It expresses the ratio of the circumference to the diameter of a circle and appears in many mathematical expressions.