# Definition:Sigma-Algebra/Definition 3

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## Definition

A **$\sigma$-algebra** $\Sigma$ is a $\sigma$-ring with a unit.

## Also known as

The term **sigma-algebra** can also be seen without the hyphen: **sigma algebra**.

Some sources refer to a **sigma-algebra** as a **sigma-field**

## Also see

## Linguistic Note

The $\sigma$ in **$\sigma$-algebra** is the Greek letter **sigma** which equates to the letter **s**.

$\sigma$ stands for for **somme**, which is French for **union**, and also **summe**, which is German for **union**.

## Sources

- 1970: Avner Friedman:
*Foundations of Modern Analysis*... (previous) ... (next): $\S 1.1$: Rings and Algebras