\begin{definition}[Definition:Neighborhood (Topology)/Set]
Let $T = \struct {S, \tau}$ be a topological space.
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:
:$\exists U \in \tau: A \subseteq U \subseteq N_A \subseteq S$
\end{definition}