\begin{definition}[Definition:Class Union]
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:
:$A \cup B = \set {x: x \in A \lor x \in B}$
\end{definition}