Definition:Star Refinement

Definition
Let $S$ be a set.

Let $\mathcal U$ be a cover for $S$.

Let $\mathcal V$ be a cover for $S$ such that:
 * $ \forall x \in S: \exists U \in \mathcal U: x^* \subseteq U$

where $x^*$ is the star of $x$ with respect to $\mathcal C$.

That is:
 * $\displaystyle x^* := \bigcup \left\{{U \in \mathcal C: x \in U}\right\}$

... the union of all sets in $\mathcal C$ which contain $x$.

Then $\mathcal V$ is a star refinement of $\mathcal U$.