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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

pis plural

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

pies plural

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  

abbreviation

1. Philippine Islands

2. private investigator

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pi ³ British  

/ paɪ /

adjective

1. slang short for pious pious

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pi ⁴ 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 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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Other Word Forms

Noun Inflected Forms

• pies
  plural
• pis
  plural

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

Sharp, who worked at the Greenwich Royal Observatory and has a crater on the moon named after him, was also a mathematician who calculated pi to 72 decimal places.

From BBC • Apr. 1, 2026

In science-fiction scenarios of first contact with extraterrestrials, humans usually bootstrap a common language with mathematics, demonstrating that we know the digits of pi and so forth.

From The Wall Street Journal • Jan. 9, 2026

The researchers discovered that the mathematical framework at the heart of Ramanujan's pi formulas also appears in the equations underlying these logarithmic conformal field theories.

From Science Daily • Dec. 16, 2025

That approaches but still falls short of the classically modeled pentacene and its 22 pi electrons.

From Science Magazine • May 30, 2024

The digits of pi will take over like an infection.

From "The Miscalculations of Lightning Girl" by Stacy McAnulty

Related Words

• inquiry agent
• private detective
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