# X

> English word · Numeral · IPA /ɛks/ · frequency rank #647

## Definitions
1. An unknown quantity or unknown value.

## Etymology
Introduced by René Descartes in the context of mathematical notation, but has since widened to other contexts.

## Easily confused with
- **xi** (https://plainspell.com/en/vs/x-vs-xi)
- **XL** (https://plainspell.com/en/vs/x-vs-xl)
- **XP** (https://plainspell.com/en/vs/x-vs-xp)
- **XO** (https://plainspell.com/en/vs/x-vs-xo)
- **XM** (https://plainspell.com/en/vs/x-vs-xm)
- **Xt** (https://plainspell.com/en/vs/x-vs-xt)
- **XC** (https://plainspell.com/en/vs/x-vs-xc)
- **XR** (https://plainspell.com/en/vs/x-vs-xr)
- **xe** (https://plainspell.com/en/vs/x-vs-xe)
- **XXX** (https://plainspell.com/en/vs/x-vs-xxx)
- **XML** (https://plainspell.com/en/vs/x-vs-xml)
- **Xie** (https://plainspell.com/en/vs/x-vs-xie)
- **XYZ** (https://plainspell.com/en/vs/x-vs-xyz)
- **Xin** (https://plainspell.com/en/vs/x-vs-xin)
- **Xia** (https://plainspell.com/en/vs/x-vs-xia)
- **XPS** (https://plainspell.com/en/vs/x-vs-xps)
- **Xue** (https://plainspell.com/en/vs/x-vs-xue)
- **ex** (https://plainspell.com/en/vs/ex-vs-x)

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