\begin{definition}[Definition:Disjoint Union (Set Theory)/Disjoint Sets]
Let $A$ and $B$ be disjoint sets, that is:
:$A \cap B = \O$
Then the disjoint union of $A$ and $B$ can be defined as:
:$A \sqcup B := A \cup B$
where $A \cup B$ is the (usual) set union of $A$ and $B$.
\end{definition}