Pi \Pi\ (p[imac]), n. [Gr. pi^.]

1. A Greek letter ([Pi], [pi]) corresponding to the Roman letter P.

2. Specif.: (Math.) The letter [Pi], [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 eight decimal places, is 3.14159265. The quotient pi cannot be expressed as a root of an algebraic equation; 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.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Pi \Pi\, n. [See Pica, Pie magpie, service-book.]

(Print.) A mass of type confusedly mixed or unsorted. [Written also pie.]

Source: Webster's Revised Unabridged Dictionary (1913)
 

Pi \Pi\, v. t. [imp. & p. p. Pied; p. pr. & vb. n. Pieing.]

(Print.) To put into a mixed and disordered condition, as type; to mix and disarrange the type of; as, to pi a form. [Written also pie.]

Source: Webster's Revised Unabridged Dictionary (1913)