\begin{definition}[Definition:Bias of Estimator]
Let $\theta$ be a population parameter of some statistical model. 
Let $\mathbf X$ be a random sample from this population.
Let $\delta$ be an estimator of $\theta$. 
The '''bias''' of $\delta$ is defined as: 
:$\map {\operatorname{bias} } \delta = \expect {\map \delta {\mathbf X} } - \theta$
\end{definition}