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• pi
  pi

  noun

  the 16th letter of the Greek alphabet (Π, π).
• PI
  PI

  personal injury.
• Pi.
  Pi.

  abbreviation

  piaster.
• P.I.
  P.I.

  abbreviation

  Philippine Islands.

pi

¹ American  

[pahy] / paɪ /

noun

plural

pis

1. the 16th letter of the Greek alphabet (Π, 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+.

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pi ² American  

[pahy] / paɪ /

Or pie

noun

plural

pies

1. printing types mixed together indiscriminately.

2. any confused mixture; jumble.

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verb (used with object)

pied, piing

1. to reduce (printing types) to a state of confusion.

2. to jumble.

PI ³ American  

1. Law. personal injury.

2. principal investigator.

3. Also P.I., p.i. private investigator.

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Pi. ⁴ American  

Or pi.

abbreviation

1. piaster.

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P.I. ⁵ American  

abbreviation

1. Philippine Islands.

2. Also PI, p.i. private investigator.

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pi ¹ British  

/ paɪ /

noun

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

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pi ² British  

/ paɪ /

noun

1. a jumbled pile of printer's type

2. a jumbled mixture

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verb

1. to spill and mix (set type) indiscriminately

2. to mix up

PI ³ British  

abbreviation

1. Philippine Islands

2. private investigator

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pi ⁴ British  

/ paɪ /

adjective

1. slang short for pious pious

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pi Scientific  

/ pī /

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.

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pi Cultural  

1. 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.

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Etymology

Origin of pi¹

First recorded in 1835–45; from Greek pî, peî, used in mathematics to represent Greek periphérion periphery ( def. )

Origin of pi²

First recorded in 1650–60; origin uncertain

Explanation

In math, pi is the ratio of a circle's circumference to its diameter. No matter how large or small a circle is, this ratio will always equal pi, or approximately 3.14. Although it sounds just like pie, this kind of pi isn't a delicious dessert with a pastry crust. Instead, it's an irrational number, one that can't be written as a simple fraction but instead goes on infinitely after the decimal point. Rounded to 3.14, it's close enough to help you calculate the diameter, radius, or area of a circle in your geometry class. Pi is an abbreviation of the Greek periphereia, "periphery."

Example Sentences

Shares, which trade under the ticker PI, dropped 5.3% to $189 ahead of the opening bell.

From Barron's • Jan. 14, 2026

Alexandre Harari is a PI in the Hi-TIDe team at Ludwig Lausanne and an associate professor at the University of Lausanne.

From Science Daily • May 7, 2024

Senior vice president at the PI, Dr Duncan French, acknowledges the UFC has a responsibility, not just to their own roster of between 650-670 fighters, but to those involved at a much lower level.

From BBC • Nov. 15, 2023

Alex Segura’s “Secret Identity,” about a Latina who invents the Legendary Lynx and moonlights as a PI, is the unlikely culmination of a life spent in love with genres.

From Los Angeles Times • Feb. 22, 2023

Just after 6:48 a.m., when the 911 call first came in, the dispatcher called out a 10-50 PI, a personal injury accident, and the coordinates.

From "A Deadly Wandering: A Mystery, a Landmark Investigation, and the Astonishing Science of Attention in the Digital Age" by Matt Richtel

Related Words

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