Definition:Class Union/General Definition

Definition
Let $A$ be a class.

The union of $A$ is:
 * $\bigcup A := \set {x: \exists y: x \in y \land y \in A}$

That is, the class of all elements of all elements of $A$ which are themselves sets.

Also denoted as
Some sources denote $\bigcup A$ as $\ds \bigcup_{x \mathop \in A} x$.