\begin{definition}[Definition:Critical Region]
Let $\theta$ be a population parameter of some population $P$. 
Let $\Omega$ be the parameter space of $\theta$. 
Let $\mathbf X$ be a random sample from $P$. 
Let $T = \map f {\mathbf X}$ be a sample statistic.
Let $\delta$ be a test procedure of the form: 
:reject $H_0$ if $T \in C$
for some null hypothesis $H_0$ and some $C \subset \Omega$.
We refer to $C$ as the '''critical region''' of $\delta$.
\end{definition}