# Definition:Neighborhood (Topology)

*This page is about neighborhoods in the context of topology. For other uses, see Definition:Neighborhood.*

## Contents

## Definition

Let $T = \left({S, \tau}\right)$ be a topological space.

### Neighborhood of a Set

Let $A \subseteq S$ be a subset of $S$.

A **neighborhood** of $A$, which can be denoted $N_A$, is any subset of $S$ containing an open set of $T$ which itself contains $A$.

That is:

- $\exists U \in \tau: A \subseteq U \subseteq N_A \subseteq S$

### Neighborhood of a Point

The set $A$ can be a singleton, in which case the definition is of the **neighborhood of a point**.

Let $z \in S$ be a point in a $S$.

A **neighborhood** of $z$, which can be denoted $N_z$, is any subset of $S$ containing an open set of $T$ which itself contains $z$.

That is:

- $\exists U \in \tau: z \in U \subseteq N_z \subseteq S$

## Also defined as

Some authorities define a neighborhood as what is defined on this site as an Open Neighborhood.

That is, in order to be a neighborhood of $A$, $N_A$ must be an open set.

However, this treatment is less common, and considered by many to be old-fashioned.

When the term **neighborhood** is used on this site, it is assumed to be not necessarily open unless so specified.

## Also see

## Linguistic Note

The UK English spelling of this is **neighbourhood**.