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Let $X$ be a topological space, let $S \subset X$, and let $x \in S$. The point $x$ is said to be an \emph{isolated} point of $S$ if there exists an open set $U \subset X$ such that $U \cap S = \{x\}$.
The set $S$ is \emph{isolated} or \emph{discrete} if every point in $S$ is an isolated point.