- https://en.wikipedia.org/wiki/Cheeger_constant_(graph_theory)
$$ \partial A := \{ (x, y) \in E(G) \ : \ x \in A, y \in V(G) \setminus A \} $$
$$h(G) := \min \left\{\frac{| \partial A |}{| A |} \ : \ A \subseteq V(G), 0 < | A | \leq \tfrac{1}{2} | V(G)| \right\}. $$
In other words, it's about finding a set of the most strongly clustered vertices and quantifying how isolated the subgraph is.