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. 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 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. 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 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

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 approach mirrors Ramanujan's own method of starting from a compact mathematical expression and rapidly arriving at precise results for pi.

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

I close my eyes and recite the numbers of pi in my head. 3.14159265358979323846264338327950288419716...

From "The Miscalculations of Lightning Girl" by Stacy McAnulty

Related Words

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