Definition:Class Union

Definition
Let $A$ and $B$ be two classes.

The (class) union $A \cup B$ of $A$ and $B$ is defined as the class of all sets $x$ such that either $x \in A$ or $x \in B$ or both:


 * $x \in A \cup B \iff x \in A \lor x \in A$

or:
 * $A \cup B = \set {x: x \in A \lor x \in B}$

Also see

 * Definition:Set Union, the usual presentation of this concept in set theory