> [!definition] > > Let $(X, \topo)$ be a [[Topological Space|topological space]] and $A \subseteq X$. The **exterior** of $A$, > $ > \exte A = \bracs{x \in X: \exists U \in \topo: x \in U \subseteq (X - A)} > $ > as the set of points that have covering [[Open Set|open sets]] completely outside $A$.