PIabbr. private investigatorpi 1 (p?)n.1. (also p?) The 16th letter of the Greek alphabet. See Table at alphabet.2. Mathematics A transcendental number, approximately 3.14159, represented by the symbol ?, that expresses the ratio of the circumference to the diameter of a circle and appears as a constant in many mathematical expressions.[Late Greek p?, from Greek pei, of Phoenician origin; see p in Semitic roots.]pi 2 also pie (p?)Printing n. pl. pis also pies An amount of type that has been jumbled or thrown together at random.v. pied (p?d), pi·ing, pies also pied, pie·ing, pies v.tr. To jumble or mix up (type).v.intr. To become jumbled.[Origin unknown.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.pi (pa?) n, pl pis1. (Letters of the Alphabet (Foreign)) the 16th letter in the Greek alphabet (?, ?), a consonant, transliterated as p2. (Mathematics) maths a transcendental number, fundamental to mathematics, that is the ratio of the circumference of a circle to its diameter. Approximate value: 3.141 592?; symbol: ?[C18 (mathematical use): representing the first letter of Greek periphereia periphery]pi (pa?) or pien, pl pies1. (Printing, Lithography & Bookbinding) a jumbled pile of printer’s type2. a jumbled mixturevb (tr) , pies, piing, pied, pies, pieing or pied3. (Printing, Lithography & Bookbinding) to spill and mix (set type) indiscriminately4. to mix up[C17: of uncertain origin]pi (pa?) adjslang Brit short for pious2, pious3PI abbreviation for 1. (Placename) Philippine Islands 2. (Law) private investigator Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014pi1 (pa?) n., pl. pis. 1. the 16th letter of the Greek alphabet (?, ?). 2. a. the letter ?, used as the symbol for the ratio of the circumference of a circle to its diameter. b. the ratio itself: 3.14159 +. [1835?45;