Definition:Set Union/Set of Sets

Definition
Let $\mathbb S$ be a set of sets.

The union of $\mathbb S$ is:
 * $\displaystyle \bigcup \mathbb S := \left\{{x: \exists X \in \mathbb S: x \in X}\right\}$

That is, the set of all elements of all elements of $\mathbb S$.

Thus the general union of two sets can be defined as:
 * $\displaystyle \bigcup \left\{{S, T}\right\} = S \cup T$