\begin{definition}[Definition:Class (Class Theory)]
A '''class''' is a collection of all sets such that a particular condition holds.
In class builder notation, this is written as:
:$\set {x: \map p x}$
where $\map p x$ is a statement containing $x$ as a free variable.  
This is read:
:'''All $x$ such that $\map p x$ holds.'''
\end{definition}