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noun, plural pis.
  1. the 16th letter of the Greek alphabet (II, π).
  2. the consonant sound represented by this letter.
  3. Mathematics.
    1. the letter π, used as the symbol for the ratio of the circumference of a circle to its diameter.
    2. the ratio itself: 3.141592+.

Origin of pi1

1835–45; < Greek pî, peî; used in mathematics to represent Greek periphérion periphery


or pie

noun, plural pies.
  1. printing types mixed together indiscriminately.
  2. any confused mixture; jumble.
verb (used with object), pied, pi·ing.
  1. to reduce (printing types) to a state of confusion.
  2. to jumble.

Origin of pi2

First recorded in 1650–60; origin uncertain


  1. Law. personal injury.
  2. principal investigator.
  3. Also P.I., p.i. private investigator.


or pi.

  1. piaster.


  1. Philippine Islands.
  2. Also PI, p.i. private investigator.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

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British Dictionary definitions for pi


noun plural pis
  1. the 16th letter in the Greek alphabet (Π, π), a consonant, transliterated as p
  2. 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: π

Word Origin

C18 (mathematical use): representing the first letter of Greek periphereia periphery



noun plural pies
  1. a jumbled pile of printer's type
  2. a jumbled mixture
verb pies, piing, pied, pies, pieing or pied (tr)
  1. to spill and mix (set type) indiscriminately
  2. to mix up

Word Origin

C17: of uncertain origin


  1. British slang short for pious (def. 2), pious (def. 3)


abbreviation for
  1. Philippine Islands
  2. private investigator
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for pi


Greek letter, from Hebrew, literally "little mouth." As the name of the mathematical constant, from 1841 in English, used in Latin 1748 by Swiss mathematician Leonhart Euler (1707-1783), as an abbreviation of Greek periphereia "periphery." For the meaning "printer's term for mixed type," see pie (3).

Online Etymology Dictionary, © 2010 Douglas Harper

pi in Medicine


n. pl. pis
  1. The 16th letter of the Greek alphabet.


  1. The pH value for the isoelectric point of a given substance in solution.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

pi in Science


  1. 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.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

pi in Culture



The irrational number obtained by dividing the length of the diameter of a circle into its circumference. Pi is approximately 3.1416. The sign for pi is π.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.