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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

None of this requires that you memorize Pi to 1,000 digits or spend weekends in memory competitions.

From The Wall Street Journal • Mar. 27, 2026

Flaherty had to wait until March 14 — Pi Day — to see if he was going to make it three for three.

From Los Angeles Times • Mar. 25, 2026

The central question for BofA economists Xiaoqing Pi and Helen Qiao: “How policymakers intend to rekindle domestic demand-both in investment and consumption,” the duo write in a note to clients.

From Barron's • Mar. 4, 2026

Raspberry Pi computers are so bare-bones that they don’t even have a case for them.

From MarketWatch • Feb. 19, 2026

Pi stares at me with his head tilted.

From "The Miscalculations of Lightning Girl" by Stacy McAnulty

Related Words

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