\begin{definition}[Definition:Power Function]
Let $\theta$ be a population parameter of some population.
Let $\Omega$ be the parameter space of $\theta$. 
 
Let $\delta$ be a test procedure for hypotheses about the value of $\theta$. 
Let $C$ be the critical region of $\delta$. 
Let $T$ be the test statistic of $\delta$. 
The '''power function''' of $\delta$, written $\map \pi {\theta_0}$, is defined by:
:$\map \pi {\theta_0} = \condprob {T \in C} {\theta = \theta_0}$
for all $\theta_0 \in \Omega$.
\end{definition}