# pi

> English word · Noun · IPA /paɪ/ · frequency rank #8,073

## Definitions
1. The sixteenth letter of the Classical and Modern Greek alphabets and the seventeenth in Old Greek.
2. An irrational and transcendental constant representing the ratio of the circumference of a Euclidean circle to its diameter; approximately 3.14159265358979323846264338327950; usually written π.

## Etymology
From Koine Greek πῖ (pî), from Ancient Greek πεῖ (peî), ultimately from Proto-Semitic *pay- (“mouth”). Doublet of pe. Its mathematical use apparently stems from its use as the first letter in περιφέρεια (periphéreia, “periphery; circumference”) and was first cited in 1706 in the Synopsis Palmariorum Matheseos by William Jones.

## Easily confused with
- **PM** (https://plainspell.com/en/vs/pi-vs-pm)
- **PP** (https://plainspell.com/en/vs/pi-vs-pp)
- **PR** (https://plainspell.com/en/vs/pi-vs-pr)
- **pt** (https://plainspell.com/en/vs/pi-vs-pt)
- **PS** (https://plainspell.com/en/vs/pi-vs-ps)
- **po** (https://plainspell.com/en/vs/pi-vs-po)
- **PL** (https://plainspell.com/en/vs/pi-vs-pl)
- **PU** (https://plainspell.com/en/vs/pi-vs-pu)
- **PK** (https://plainspell.com/en/vs/pi-vs-pk)
- **PV** (https://plainspell.com/en/vs/pi-vs-pv)
- **PJ** (https://plainspell.com/en/vs/pi-vs-pj)
- **PW** (https://plainspell.com/en/vs/pi-vs-pw)
- **PX** (https://plainspell.com/en/vs/pi-vs-px)
- **put** (https://plainspell.com/en/vs/pi-vs-put)
- **pro** (https://plainspell.com/en/vs/pi-vs-pro)
- **pop** (https://plainspell.com/en/vs/pi-vs-pop)
- **pot** (https://plainspell.com/en/vs/pi-vs-pot)
- **pin** (https://plainspell.com/en/vs/pi-vs-pin)
- **pit** (https://plainspell.com/en/vs/pi-vs-pit)
- **pub** (https://plainspell.com/en/vs/pi-vs-pub)

## Source
Compiled from Wiktionary via kaikki.org (CC BY-SA). Data vintage: 2026-05-06.
Canonical page: https://plainspell.com/en/word/pi