\begin{definition}[Definition:Empty Class (Class Theory)]
A class is defined as being '''empty''' {{iff}} it has no elements.
That is:
:$\forall x: x \notin A$
or:
:$\neg \exists x: x \in A$
The '''empty class''' is usually denoted $\O$ or $\emptyset$.
On {{ProofWiki}} the preferred symbol is $\O$.
\end{definition}