# PI

/ˈpaɪ/ (US) · /pˈaɪ/ (UK) · CEFR B2 · Noun

Source: https://englishreference.com/word/pi
Updated: 2026-10-07

## Definition

a mathematical number that is roughly 3.14. You use it to calculate the area or the distance around a circle.

For teachers: a mathematical constant representing the ratio of a circle's circumference to its diameter, approximately equal to 3.14159.

## Examples

- We used pi to find the area of the circle.
- The students were asked to calculate the volume of the cylinder using pi to two decimal places.
- Because pi is an irrational number, its decimal representation never ends and never settles into a repeating pattern, a property that has fascinated mathematicians for centuries.

## Usage notes

- Usually written as the Greek letter π in mathematical equations.

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